Illuminator assembly incorporating light emitting diodes

ABSTRACT

An illuminator assembly, having a plurality of LEDs on a vehicular support member in a manner such that, when all of the LEDs are energized, illumination exhibiting a first perceived hue, e.g., blue-green, and projected from at least one of the LEDs overlaps and mixes with illumination exhibiting a second perceived hue, e.g., amber, which is distinct from said first perceived hue and which is projected from at least one of the remaining LEDs in such a manner that this overlapped and mixed illumination forms a metameric white color and has sufficient intensity and color rendering qualities to be an effective illuminator.

The present invention relates to an illuminator assembly incorporatinglight emitting diodes, and more particularly to vehicular, portable andother specialty white light illumination systems utilizing lightemitting diodes having complementary hues.

BACKGROUND OF THE INVENTION

Due to limitations in human vision in low light level environments,white light illuminator systems have long been used to produceartificial illumination and enhance visibility during nighttime orovercast conditions or within interior quarters obscured from the reachof solar illumination. Illuminators are therefore generally designed tomimic or reproduce daytime lighting conditions, to the extent possible,so that illuminated subjects of interest are bright enough to be seenand have sufficient visual qualities such as color and contrast to bereadily identifiable.

A diversity of illuminator systems such as stationary lamps inbuildings, portable flashlights, and vehicular headlamps and courtesylights have evolved throughout history and have traditionally producedwhite light for general, spot or flood illumination, using a variety ofsources such as candles, oil, kerosene and gas burning elements,incandescent and halogen bulbs, and fluorescent and other arc-dischargelamps. White light is critical in such uses because of its uniqueability to properly render colored objects or printed images relative toone another and its similarly unique ability to preserve luminance andcolor contrast between adjacent objects or printed images havingdifferent colors. For instance, a blue photographic image of an oceanpanorama will be readily distinguished by an unaided observer from blackphotographic images of volcanic rocks when the photograph containingthese images is illuminated by white light. The two images would,however, be virtually indistinguishable from one another if illuminatedwith a deeply red colored illuminator. Another example arises from theneed to properly identify differently-colored regions on conventionalaeronautical or automotive maps. On an automotive map, white lightilluminators make it easy to discern the difference between the yellowmarkings for urban regions and the surrounding white rural areas. Adeeply yellow colored illuminator would make this distinction virtuallyimpossible. On an aeronautical chart, white light illuminators make itpossible to discern the difference between the characteristic bluemarkings for certain types of controlled airspace and the green patternof underlying terrain, whereas a deeply red colored illuminator wouldmake this distinction virtually impossible.

Furthermore, these issues of color discrimination and contrast go beyondthe simple need for accurate identification. It is, for example, a wellknown fact that high contrast is critical for avoiding severe operatoreye fatigue and discomfort during prolonged visual tasks, whether thesubject of study is a book, magazine, newspaper or a map. White lightilluminators provide more universally high contrast and good colordiscrimination, thereby avoiding these annoying and dangerousphysiological side effects.

The extensive evolution and widespread use of white light illuminators,along with rapidly advancing technology and a phenomenon known as colorconstancy, have fostered acceptance of a rather broad range ofunsaturated colors as "white". Color constancy refers to the well-knownfact that the level and color of slightly unsaturated or near-whiteillumination over an area can vary moderately without substantiallyaltering the perceived colors of objects in that setting relative to oneanother. An example of this is the appearance of an outdoor scene to anobserver wearing slightly amber or green sunglasses. After a briefmoment of adaptation upon donning the sunglasses, an observer becomesunaware that the scene is being passed through a slightly coloredfilter. Another example is the tacit acceptance of a wide variety of"white" illuminators in residential, commercial, and publicillumination. The bluish or cool white from various fluorescent lamps isvirtually universal in office buildings, whereas the yellowish or warmwhite of incandescent lamps is dominant in residential lighting. Thebrilliant bluish-white of mercury vapor and metal halide lamps iscommonplace in factory assembly lines, whereas the bronze-white emissionof the high pressure sodium lamp dominates highway overhead lighting inurban areas. Despite the discernible tint of each of these sources whichwould be evident if they were compared side by side, they are generallyaccepted as white illuminators because their emissions are close enoughto an unsaturated white to substantially preserve relative colorconstancy in the objects they illuminate. In other words, they renderobjects in a manner that is relatively faithful to their apparent "true"colors under conditions of natural illumination.

There are limits to the adaptability of human color vision, however, andcolor constancy does not hold if highly chromatic illuminators are usedor if the white illumination observed in a setting is altered by astrongly colored filter. A good example of this limitation can beexperienced by peering through a deeply colored pair of noveltysun-glasses. If these glasses are red, for instance, then it will benearly impossible to discern a line of red ink on white paper, eventhough the line would stand out quite plainly in normal roomillumination if the glasses are removed. Another illustration of thiseffect is the low-pressure sodium lamp used for certain outdoor urbanillumination tasks. This type of lamp emits a highly saturated yellowlight which makes detection and or identification of certain objects orprinted images very difficult if not impossible, and, consequently theircommercial use has been very limited. As will be discussed later, asimilar problem arises from prior-art attempts to use high intensity redor amber light emitting diodes (LEDs) as illuminators since they, likethe low-pressure sodium lamp, emit narrow-band radiation without regardfor rendering quality.

In order to improve the effectiveness of white light illuminationsystems, various support structures are typically employed to containthe assembly and provide energy or fuel to the incorporated light sourcetherein. Furthermore, these systems typically incorporate an assortmentof optical components to direct, project, intensify, filter or diffusethe light they produce. A modern vehicle headlamp assembly, forinstance, commonly includes sealed electrical connectors, sophisticatedinjection-molded lenses and molded, metal-coated reflectors which workin concert to collimate and distribute white light from an incandescent,halogen, or arc-discharge source. A backlight illuminator for aninstrument panel in a vehicle or control booth typically containselaborate light pipes or guides, light diffusers and extractors.

Of course, traditional white light sources which generate light directlyby fuel combustion are no longer suitable for most vehicular,watercraft, aircraft, and portable and certain other applications wherean open flame is unsafe or undesirable. These have therefore have beenalmost universally superseded by electrically-powered, white lightsources. Furthermore, many modem electric light sources are relativelyinefficient, e.g., conventional tungsten incandescent lamps, or requirehigh voltages to operate, e.g., fluorescent and the gas discharge lamps,and therefore aren't optimal for vehicular, portable, and other uniqueilluminators used where only limited power is available, only lowvoltage is available or where high voltage is unacceptable for safetyreasons.

Because no viable alternatives have been available, however,illuminators for these overland vehicles, watercraft, aircraft and theother fields mentioned have used low-voltage incandescent white-lightilluminators for quite some time to assist their operators, occupants,or other observers in low light level situations. In automobiles,trucks, vans and the like, white light illuminators are used as domelights, map lights, vanity mirror lights, courtesy lights, headlamps,back-up lights and illuminators for the trunk and engine compartmentsand license plate. In such vehicles, white light illuminators are alsoused to backlight translucent screen-printed indicia such as those foundin an instrument cluster panel, door panel, or heater and ventilationcontrol panel. Similar uses of white light incandescent illuminators arefound on motorcycles, bicycles, electric vehicles and other overlandcraft. In aircraft, white-light illuminators are used in the passengercompartment as reading lamps, to illuminate the floor and exits duringboarding, disembarking, and emergencies, to illuminate portions of thecockpit, and to back-light or edge-light circuit breaker panels andcontrol panels. In water-craft such as ships, boats and submarines,white-light illuminators are used to illuminate the bridge, the decks,cabins and engineering spaces. In portable and specialty lightingapplications, low-voltage white light illuminators are used ashand-held, battery-powered flashlights, as helmet-mounted orhead-mounted lamps for mountaineering or mining, asautomatically-activated emergency lighting for commercial buildings, astask lighting in volatile environments, and as illuminators in a widevariety of other situations where extreme reliability, low voltage,efficiency and compactness are important.

These aforementioned white-light illuminators rely almost exclusivelyupon incandescent lamps as light sources because incandescent bulbs areinexpensive to produce in a wide variety of forms and, more importantly,they produce copious quantities of white light. Despite this,incandescent lamps possess a number of shortcomings which must be takeninto account when designing an illuminator assembly.

Incandescent lamps are fragile and have a short life even in stableenvironments and consequently must be replaced frequently at greatinconvenience, hazard, and/or expense. This need for replacement hascomplicated designs for all manner of illuminators, but especially forvehicles. For example, in U.S. Pat. No. 4,087,096, Skogler et al.disclose a carrier module for supporting lamps for illuminating aportion of a vehicle interior. The carrier module has a rigid body and apair of mounting projections for removably mounting the carrier modulein a rearview mirror. The design even has an opening specificallydesigned to allow insertion of a tool for releasing the module from therearview mirror. This carrier module is an excellent example of theHerculean design efforts taken by mirror manufactures to ensureincandescent lamps can be easily removed and replaced by a vehicleowner.

In addition to their inherently short life, incandescent lamps are verysusceptible to damage from mechanical shock and vibration. Automobilesexperience severe shocks and significant vibration during drivingconditions which can cause damage to incandescent lamps, particularlythe filaments from which their light emissions originate. This is anespecially severe problem for lamps mounted on or near the engine hood,trunk lid, passenger doors, exterior mirrors, and rear hatch or gate,all of which periodically generate tremendous shocks periodically uponclosing. Aircraft and portable illuminators experience similarenvironments, and therefore another source of white light would behighly beneficial to decrease the time and cost associated withreplacing lamps therein on a regular interval.

Incandescent lamps can also be easily destroyed by exposure to liquidmoisture due to the thermo-mechanical stress associated with contactbetween the hot glass bulb wall and the room-temperature fluid.Incandescent lamps are also easily damaged by flying stones and thelike. Thus, it is very difficult to incorporate an incandescent light onan exterior mirror without going to extreme measures to protect thelight bulb from shock, vibration, moisture and flying objects whilestill allowing for removal of the light fixture when it either burns,out or is otherwise permanently damaged.

Incandescent lights also exhibit certain electrical characteristicswhich make them inherently difficult to incorporate in vehicles, such asan automobile. For instance, when an incandescent light source is firstenergized by a voltage source, there is an initial surge of currentwhich flows into the filament. This in-rush current, which is typically12 to 20 times the normal operating current, limits the lifetime of thelamp thus further amplifying the need for a illuminator structure whichallows for frequent replacement. Inrush current also necessitatesunusual consideration when designing supporting electrical circuitswhich contain them. Fuses, relays, mechanical or electronic switches,wire harnesses, and connectors electrically connected to such lamps mustbe capable of repeatedly carrying this extreme transient.

In addition, the voltage-current (V-I) characteristic of incandescentlamps is notoriously non-linear, as are each of the relationshipsbetween light output and voltage, current, or power. The luminousintensity, color temperature, and service life of incandescent lampsvaries exponentially as a function applied current or voltage. Thissensitivity to power source variation makes electronic control ofincandescent lamps a particularly difficult problem. They are furthersusceptible to significant reliability and field service lifedegradation when subjected continuously to DC electrical power,pulse-width modulated DC power, simple on/off switching of any sort, orany over-voltage conditions, however minor. Incandescent lamps alsopossess significant inductance which, when combined with theirrelatively high current load, complicates electronic switching andcontrol greatly due to inductive resonant voltage transients. A typicalsquare wave, DC pulse modulation circuit for a 0.5 amp, 12.8 voltincandescent lamp might produce brief transients as high as 30 volts,for instance, depending on the switching time, the lamp involved, andthe inductance, capacitance, and resistance of the remainder of thecircuit.

Incandescent lamps also suffer from poor efficiency in convertingelectrical power into radiated visible white light. Most of theelectrical energy they consume is wasted in the form of heat energywhile less than 7% of the energy they consume is typically radiated asvisible light. This has severe negative consequences for vehicular,aerospace, watercraft, and portable illuminator applications where theamount of power available for lighting systems is limited., In theseapplications, electrical power is provided by batteries which areperiodically recharged by a generator on a ship or aircraft, analternator in an automobile, by solar cells in the case of some remoteor aerospace applications, or are otherwise periodically replaced orrecharged with an AC/DC adapter such as in the case of a flashlight.Because these mechanisms for restoring battery charge are inherentlybulky, heavy, and/or expensive, it is severely detrimental for anilluminator to possess poor power-conversion efficiency in generatingvisible light. An acute example of the importance in illuminatorefficiency is the electric vehicle. For electric bicycles, mopeds,motorcycles, automobiles, golf carts, or passenger or cargo transfercarts, white-light illuminators in the form of electric headlamps,backup lamps, etc. consume an unusually large portion of the vehicle'slimited power budget; hence they would benefit greatest fromhigh-efficiency white-light illuminators. If a more efficientwhite-light source was available, much less power would be required toenergize the illuminator and more power would be available for othersystems. Alternatively, the power savings from an improved illuminatorwould allow for improved power supplies and energy storage or energyreplacement mechanisms.

Another resultant of poor efficiency associated with incandescent lampsis that they generate large amounts of heat for an equivalent amount ofgenerated light as compared to other sources. This results in very highbulb-wall temperatures typically in excess of 250 degrees C and largeheat accumulations which must be dissipated properly by radiation,convection, or conduction to prevent damage or destruction to theilluminator support members, enclosure, optics or to other nearbyvehicle components. This high heat signature of common incandescentlight sources in illuminators has a particularly notable impact on thespecialized reflector and lens designs and materials used to collimateand direct the light. Design efforts to dissipate the heat whileretaining optical effectiveness further add requirements for space andweight to the illuminator assembly, a severe disadvantage for vehicular,watercraft, aircraft and portable applications which are inherentlysensitive to weight and space requirements.

Portable illuminators such as hand-held flashlights and head-mountedlamps experience similar problems; stemming from incandescentwhite-light sources and would derive the same benefits from an improvedsystem.

Physical mechanisms for generating white-light radiation other thanincandescence and pyroluminescence are available, including various gasdischarges, electroluminescence, photoluminescence, cathodoluminescence,chemiluminescence and thermoluminescence. The output of sources usingthese phenomena can be tailored to meet the requirements of specificsystems; however, they have had limited use in vehicular, watercraft,aircraft or portable illuminators because of a combination of lowintensity, poor efficiency, high voltage requirements, limitedenvironmental resilience, high weight, complexity, high cost, poorreliability, or short service life.

More recently, great interest has been shown in the use ofelectroluminescent semi-conductor devices such as light emitting diodes(LEDs) as the light source for illuminator systems. Due to their strongcoloration and relatively low luminous output as compared toincandescent lamps, early generations of LEDs found most of theirutility as display devices, e.g., on/off and matrix-addressedindicators, etc. These uses still dominate the LED market today, howeverrecent advances in LED materials, design and manufacturing have resultedin significant increases in LED luminous efficacy and, in their mostrecent commercial forms, exhibit a higher luminous efficacy thanincandescent lights. But even the latest LEDs emit highly-saturated,narrow-bandwidth, distinctively non-white light of various hues. Asdiscussed above, white light in one of its various manifestations isessential for most illuminator systems.

Despite the inherent colorfulness of LEDs, they offer many potentialadvantages as compared to other conventional low voltage light sourcesfor vehicles, watercraft, aircraft and portable illuminators. LEDs arehighly shock resistant and therefore provide significant advantages overincandescent and fluorescent bulbs which can shatter when subjected tomechanical or thermal shock. LEDs possess operating lifetimes from200,000 hours to 1,000,000 hours, as compared to the typical 1,000 to2,000 hours for incandescent lamps or 5,000-10,000 hours forfluorescent.

It has been known that the narrow-band spectral emissions of severalsaturated light sources having different apparent colors can be combinedto produce an additive color mixture having an apparent color which isdifferent than that of any of its constituents. The basics of additivecolor are evident, for instance, in the observation that white sunlightdecomposes into its constituent spectra when refracted by a prism ordispersions of water droplets such as occurs in a typical rainbow. Thevisible white light of the sun can therefore be considered an additivecolor mixture of all of the hues associated with its radiation in thevisible spectrum having wavelengths from 380 to 780 nanometers.

An important and common example of additive color mixtures is thetechnique used in most color display screens possessing a cathode raytube (CRT) or a liquid crystal display (LCD) element. These displaysconsist of addressable arrays of pixels, each of which containssub-pixels having the hues red, green and blue which can be energizedalone or in combinations. In the case of the CRT, each sub-pixel is adot of inorganic phosphor which can be excited via cathodoluminescenceby a steered electron beam. In the case of the LCD, each sub-pixel is adot of colored dye in registry with a switchable liquid crystal shutter,the combination of which acts as a reconfigurable filter for abacklight. The result in either of these cases is that a brightlycolored red sub-pixel can be energized simultaneously with an adjacentbright green pixel in unresolvable proximity to the red in order to formthe perceived color yellow. A similar combination of the green sub-pixeland a blue one will form the perceived color cyan. A similar combinationof the red sub-pixel and a blue one will form the perceived colormagenta. Energizing all three of the red, green, and blue sub-pixelswithin a pixel concurrently will yield the perceived color white, if thebrightness of each sub-pixel is proportioned properly. The relativeproportions of the brightness of each of these differently coloredsub-pixels can further be actively manipulated in a wide variety ofcombinations resulting in a continuum of perceived colors nearlyreplicating all of the colors available within human color vision,including white. Unfortunately, while these types of displays mayexhibit appreciable surface brightness, they are extremely bulky,expensive and complicated and do not project suitable amounts ofillumination at a distance to be of use as effective illuminators. Forexample, even the brightest and largest television screen casts only adim glow across a darkened room. The illumination level associated withthis dim glow is barely sufficient for reading a newspaper and iscompletely inadequate to identify objects or colors in a detailedphotograph. However, the capability of such an R-G-B display system toreproduce appreciably all of the colors available within human colorvision is an excellent example of the important phenomenon known asmetamerism, which will be discussed in greater detail hereinafter.

LEDs are available in various hues and it is known that the output ofred, blue and green LEDs can be combined in a fashion similar to thatused for a CRT in the proper proportions to produce a variety ofperceived colors, including the perceived color white. For example, inU.S. Pat. No. 5,136,483, Karl-Heinz Schoniger et al. disclose a lightemitting device having twelve LEDs arranged to form a headlamp orsignaling lamp. Schoniger et al. also disclose that to produce whitelight, red, green and blue LEDs need to be used simultaneously. However,such as system is rather complicated and Schoniger et al. do not mentionthe inherent susceptibility of an R-G-B system to unacceptable variationdue to significant variations in luminous output produced from one LEDto another of the same type. Such LED variations causes errors in therelative proportions of the actual color mixture produced versus thatdesired and, coupled with high complexity and cost, render the systemundesirable for most practical uses.

Consequently, it is desirable to provide a highly reliable, low-voltage,long-lived, LED illuminator capable of producing white light withsufficient luminous intensity to illuminate subjects of interest wellenough to be seen and to have sufficient apparent color and contrast soas to be readily identifiable.

OBJECTS OF THE INVENTION

Accordingly, a primary object of the present invention is to provide anilluminator assembly projecting effective white illumination and havinga plurality of LEDs of two types whose visible emissions when energizedhave hues which are complementary to one another and combine to form ametameric white illumination.

Another object of the present invention is to provide a high efficiencyilluminator assembly, for use in limited power applications, projectingeffective white illumination and having a plurality of LEDs of two typeswhose visible emissions when energized have hues which are complementaryto one another and additively combine to form illumination with ametameric white color.

Yet another object of the present invention is to provide an automotiverearview mirror incorporating an illuminator assembly projectingeffective white illumination and having a plurality of LEDs of two typeswhose visible emissions when energized have hues which are complementaryto one another and whose beams overlap and additively mix to form ametameric white illumination.

Yet another object of the present invention is to provide an illuminatorassembly projecting an effective photopic white illumination within acentral zone and mesopic illumination in a surrounding zone bounded fromthe first by a Photopic illuminance threshold and having a plurality ofLEDs of two groups or types whose emissions when energized form anadditive binary complementary or equivalent binary complementary colormixture.

Still another object of the present invention is to provide a circuitoperable to power an illuminator assembly of the present invention.

SUMMARY OF THE INVENTION

The above and other objects, which will become apparent from thespecification as a whole, including the drawings, are accomplished inaccordance with the present invention by disposing a plurality of lightemitting diodes on a support member to provide a light-weight, robustilluminator.

Briefly, according to a preferred embodiment of the invention, anilluminator assembly is provided by placing on a support member in ahousing a plurality of LEDs of two types whose visible emissions whenenergized have hues which are complementary to one another e.g.,blue-green and amber, and are projected such that their overlapped andmixed beams form a metameric white illumination having sufficientintensity and color rendering qualities to be effective.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings, wherelike numerals represent like components, in which:

FIG. 1 is a cross-sectional view of an illuminator assembly according tothe present invention incorporating conventional discrete LEDs;

FIG. 2 is a cross-sectional view of an illuminator assembly according tothe present invention incorporating a plurality of LED chips in achip-on-board configuration;

FIG. 3 is a graph plotting the relative spectral power versus wavelengthfor Standard Illuminants A, B and C, as well as amber and blue-greenLEDs;

FIGS. 4a, 4b and 4c are a series of graphs plotting the relativespectral power versus wavelength for amber and blue-green LEDs, thespectral reflectance versus wavelength for a 50% neutral gray target andthe relative spectral power versus wavelength for the resultantreflected light, respectively;

FIG. 5 is a graph plotting the relative sensitivity of a standard twodegree observer versus wavelength for photopic vision, scotopic visionas well as an estimated mesopic vision;

FIG. 6 is a graph plotting the relative response of the color-matchingfunctions for a standard two degree observer versus wavelength duringphotopic vision;

FIG. 7 is a CIE 1976 uniform chromaticity scale (UCS) diagram showingthe location of the Planckian Locus, the location of the translated SAEJ578 boundaries for achromatic white light, the locations of CIEStandard Illuminants A, B and C as well as the locus of binary additivemixtures from blue and red LEDs.

FIG. 8 is a CIE 1976 UCS diagram showing the location of the PlanckianLocus, the locations of Standard Illuminants A, B and C, the location ofthe translated SAE J578 boundaries for achromatic white light, the locusof ternary additive mixtures from red, green and blue LEDs, as well asan estimated locus of red, green, blue LED manufacturing variation;

FIG. 9 is a CIE 1976 UCS diagram showing the location of the PlanckianLocus, the locations of Standard Illuminants A, B and C, the location ofthe translated SAE J578 boundaries for achromatic white light, and theapproximate location of the white color boundary translated from therevised Kelly chart, as well as the locus of binary additive mixturesfrom deep red and deep green LEDs;

FIG. 10 is a CIE 1976 UCS diagram showing the location of the PlanckianLocus, the locations of Standard Illuminants A, B and C, the location ofthe translated SAE J578 boundaries for achromatic white light, and thelocus of binary additive mixtures from amber 592 nm and blue-green 488nm LEDs which is substantially coaxial with the Planckian Locus;

FIG. 11 is a CIE 1976 UCS diagram showing the location of the PlanckianLocus, the locations of Standard Illuminants A, B and C, the location ofthe translated SAE J578 boundaries for achromatic white light, theapproximate location of the white color boundary translated from therevised Kelly chart, and the locus of binary additive mixtures from arange of amber and blue-green LEDs which is substantially coaxial withthe Planckian Locus;

FIG. 12 is a CIE 1976 UCS diagram showing the location of the PlanckianLocus, the locations of Standard Illuminants A, B and C, the location ofthe translated SAE J578 boundaries for achromatic white light, and thelocus of binary additive mixtures from amber 584 nm and blue-green 483nm LEDs which is substantially coaxial with the Planckian Locus;

FIG. 13 is a CIE 1976 UCS diagram showing the locations of StandardIlluminants A, B and C, the location of the translated SAE J578boundaries for achromatic white light, and the locus of binary additivemixtures from equivalent 584 nm amber and equivalent 483 nm blue-greenLEDs which is substantially coaxial with the Planckian Locus;

FIGS. 14a, is a perspective view of an automotive interior rearviewmirror incorporating the illuminator assembly of the present invention,and FIGS. 14b and 14c are cross-sectional views of exemplary mirrorelements for insertion into the rearview mirror;

FIG. 15 is an illustration of a specification for an area targeted byillumination from an automotive interior rearview mirror maplight;

FIG. 16 is an illustrative illumination pattern for an illuminatorassembly according to the present invention;

FIG. 17 is a perspective three-dimensional chart plotting the intensitydistribution from an illuminator maplight according to the presentinvention borne by an automotive interior electrochromic mirror;

FIG. 18 is an iso-intensity contour chart plotting the intensitydistribution from an illuminator maplight according to the presentinvention borne by an automotive interior electrochromic mirror;

FIG. 19 is an iso-illuminance contour chart plotting the illuminationpattern at a target from an illuminator maplight according to thepresent invention borne by an automotive interior electrochromic mirror;

FIG. 20 is a contour map plotting the surface luminance of a 50% neutralgray target illuminated by an illuminator maplight according to thepresent invention borne by an automotive interior rearview mirror;

FIG. 21 is a schematic diagram of an electronic circuit operable topower the illuminator assembly of the present invention; and

FIG. 22 is a plot of the specified maximum forward current versustemperature for a typical LED and the experimentally determined forwardcurrent versus temperature plot for the LEDs of the present inventionoperated by the circuit of FIG. 21, as well as the design current versustemperature plot for the LEDs of the present invention operated by thecircuit of FIG. 21 which also incorporates microprocessor softwarecontrols.

DETAILED DESCRIPTION

The present invention generally relates to an improved illuminator andmore specifically to a white-light LED illuminator for use in limitedpower applications such as vehicles, portable lamps, and specialtylighting. By vehicles we mean over-land vehicles, watercraft, aircraftand manned spacecraft, including but not limited to automobiles, trucks,vans, buses, recreational vehicles (RVs), bicycles, motorcycles andmopeds, motorized carts, electric cars, electric carts, electricbicycles, ships, boats, hovercraft, submarines, airplanes, helicopters,space stations, shuttlecraft and the like. By portable lamps, we meancamping lanterns, head or helmet-mounted lamps such as for mining,mountaineering, and spelunking, hand-held flashlights and the like. Byspecialty lighting we mean emergency lighting activated during powerfailures, fires or smoke accumulations in buildings, microscope stageilluminators, billboard front-lighting, backlighting for signs, etc.

The present invention provides a highly reliable, low-voltage,long-lived, LED illuminator for vehicles, portable lighting andspecialty lighting capable of producing white light with sufficientluminous intensity to illuminate subjects of interest well enough to beseen and to have sufficient apparent color and contrast so as to bereadily identifiable. The LEDs of the present invention exhibitextremely predictable electronic properties and are well suited for usewith DC power sources, pulse-width modulated DC power sources, andelectronic control systems. LEDs do not suffer appreciable reliabilityor field-service life degradation when mechanically or electronicallyswitched on and off for millions of cycles. The luminous intensity andilluminance from LEDs closely approximates a linear response functionwith respect to applied electrical current over a broad range ofconditions, making control of their intensity a relatively simplematter. Finally, recent generations of AlInGaP, AlGaAs, and GaN LED'sdraw less electrical power per lumen or candela of visible lightproduced than incandescent lamps, resulting in more cost-effective,compact, and lightweight illuminator wiring harnesses, fuses,connectors, batteries, generators, alternators, switches, electroniccontrols, and optics. A number of examples have previously beenmentioned and are incorporated within the scope of the presentinvention, although it should be recognized that the present inventionhas obvious other applications beyond the specific ones mentioned whichdo not deviate appreciably from the teachings herein and therefore areincluded in the scope of this invention.

FIGS. 1 and 2 show two embodiments of the present invention using LEDsof two substantially different configurations; FIG. 1 shows anembodiment incorporating conventional discrete LEDs, and FIG. 2 shows anembodiment incorporating individual LED chips.

Conventional discrete LED components include such LED devices such asT1, T 13/4, T5, surface mount (SMD), axial-leaded "polyleds", and highpower packages such as the SuperNova, Pirahna, or Brewster lamps, all ofwhich are available with a variety of options known to those skilled inthe art such as color, size, beam width, etc. Appropriate conventionaldiscrete LEDs may be obtained from manufacturers such as HewlettPackard, Inc., Optoelectronics Division, located in San Jose, Calif.,Stanley Electric Company, LTD located in Tokyo, Japan, Nichia ChemicalIndustries, LTD located in Anan-shi, Tokushima-ken, Japan and manyothers.

Conventional discrete LEDs 14 are the dominant form of LEDs in generaluse because of their generic shapes and ease of processing in standardprinted circuit board assembly operations. Referring to FIG. 1,illuminator 10 is shown including a support member 12 which supports,delivers electrical power to, and maintains a spatial relationshipbetween a plurality of conventional discrete LEDs 14. The structure ofsupport member 12 will vary depending on the specific design of the LEDs14 and of the illuminator 10, and may be a conventional printed circuitboard or optionally may be a portion of housing 19 into which theilluminator assembly 10 is being incorporated. Support member 12 may beshaped such that the emission of all of the LEDs is aligned or otherwisefocused on a common spot at some predetermined distance away from theilluminator 10. A conventional discrete LED 14 generally consists of apre-assembled or packaged "lamp" each of which normally includes a metallead frame 17 or other substrate for electrical and mechanicalconnection and internal mechanical support, a semiconductor LED chip or"die" 16, a conductive adhesive or "die attach" (not shown) forelectrically and mechanically attaching one electrode of the chip 16 tothe lead frame 17 or other substrate, a fine wire conductor 20 forelectrically connecting the other electrode of the chip 16 to the anarea of the lead frame 17 or other substrate which is electricallyisolated from the first electrode and die attach by the chip 16 itself.Optionally, a miniature reflector cup (not shown) may also be locatedadjacent to the chip 16 to further improve light extraction from thedevice. Finally, a clear, tinted, or slightly diffused polymer matrixenclosure 18 is used to suspend, encapsulate, and protect the chip 16,lead frame 17, optional reflector cup (not shown) and wire conductor 20and to provide certain desirable optical characteristics.

In conventional discrete LEDs 14, the polymer matrix enclosure 18typically comprises an optically clear epoxy or any number of materialscapable of protecting the LED chip 16 and an upper portion of lead frame17 from environmental contaminants such as moisture. As shown in FIG. 1,polymer matrix enclosure 18 can further be made integral with lens 27which will be discussed in greater detail hereinbelow. The upper portionof lead frame 17 is connected to the LED semiconductor chip 16 and alower portion of lead frame 17 extends out one end of the enclosure 18to attach to support member 12 and provide electrical connection to anelectronic control circuit 22 through wires 23. Circuit 22 is operableto energize, control and protect the LEDs 14, and manipulate and managethe illumination they produce. Many variations of electronic controlcircuit 22 will be known to those skilled in the art and will varydepending on the application for illuminator 10. For example, electroniccontrol circuit 22 for a flashlight may simply be an ON-OFF switch, abattery and a resistor in series with the LEDs 14 and support member 12.However, for an automotive rearview mirror assembly, described in detailhereinbelow, circuit 22 will be slightly more complex.

In most conventional discrete LED designs, enclosure 18 also acts as anintegral optical element such as a lens 27, deviator 28 or diffuser 29,however separate or secondary optical elements 21 are preferablyincorporated in illuminator 10 to improve illuminator performance orappearance. Furthermore, more than one individual LED chip 16 of thesame color or of different colors may be incorporated within a singlepolymer matrix enclosure 18 such that the spacing between conventionaldiscrete LEDs 14 is greater than the spacing between individual chips16.

A second configuration of LEDs is the individual LED chip, consistingsolely of a semiconductor LED chip (without a pre-attached lead frame,encapsulating media, conducting wire, etc.). These are generally shippedin vials or adhered to a membrane called "sticky back" and are mountedin an intermediate manufacturing step directly onto a printed circuitboard, ceramic substrate, or other structure to support the individualLED chip and provide electrical connections to it. When a plurality ofLEDs is so mounted, the result is a "chip-on-board" LED array which inits entirety can then be incorporated into other assemblies as asubcomponent. Individual LED chips suitable for the present inventionare available from Hewlett Packard, Showa Denko, Stanley, and CreeResearch, to name just a few. Referring to FIG. 2, if chip-on-board LEDdesigns are utilized, then illuminator 10 has a support member 12 whichmay be a printed circuit board, ceramic substrate, housing or otherstructure capable of supporting the individual LED chips 16 whilesimultaneously providing electrical connection for powering the chips16. In this configuration, individual LED chips 16 are placed on supportmember 12, thereby eliminating the bulky pre-packaged polymer matrixenclosure 18, and lead frame 17 of the conventional discrete type of LED14 in FIG. 1. A more integrated and optimized system is thereforepossible by virtue of the flexibility to place individual LED chips 16within very close proximity to one another on the support member 12 andwithin very close proximity to reflector 26, lens 27, and/or secondaryoptical elements 21 used to enhance the light emissions of LED chip 16.In this manner one or more LED chips 16 can be placed at or very near tothe focus of a single lens 27 or lenslet 27a (as shown in areas A andB), improving the alignment and uniformity of the resultant mixed beamprojected therefrom. Individual LED chips 16 are very small (on theorder of 0.008 inches ×0.008 inches×0.008 inches) and can be placed veryclosely to one another by precision equipment, e.g., pick-and-placemachines. Such close pitch spacing is not possible with the conventionaldiscrete LEDs 14 of FIG. 1 because of their relatively large size andlarger tolerances associated with their manufacture and assembly.Furthermore, the ability to tightly pack the chips 16 allows extremedesign flexibility improving the aesthetic appeal of illuminator 10.

For chip-on-board designs, the individual LED chips 16 are electricallyconnected to conductive pad 24 by a fine conductive wire 20 and attachedto conductive pad 25 by an electrically conductive die attach adhesive(not shown). The chips 16 and conductive pads 24 and 25 are mounted on,and held in a spaced apart relationship from one another, by supportmember 12. LED chips 16 are electrically connected to the support member12, and to electronic circuit 22, through pads 24 and 25, support member12 and wires 23.

Referring to Areas A and B, the number, spacing, color and pattern ofindividual LED chips 16 under each lenslet 27a can vary from system tosystem. One or more chips 16 of the same color or different colorschosen according to the teachings of this invention may be placed undera single lenslet 27a such that the spacing between groups of LED chipsis greater than the spacing between individual chips. For instance, inArea A, two of the three individual LED chips 16 shown may be a typethat emit amber light when energized and the third may be of a typewhich emits blue-green light when energized. Alternatively, two may beof the blue-green variety and one may be of the amber variety. Also, itis possible for all of the LEDs in Area A to be of one color, e.g.amber, if another nearby group in the plurality of the illuminator suchas that shown in Area B of FIG. 2 contains an appropriate number ofcomplementary LEDs, e.g. two of the blue-green variety.

A reflector 26 may optionally be used with the above describedconventional discrete LED designs as shown in FIG. 1 or with LED arraychip-on-board designs shown in FIG. 2. The reflector 26, if used, isnormally a conical, parabolic, or elliptical reflector and typically ismade of metal or metal-coated molded plastic. The purpose of thereflector 26 is to collect or assist in the collection of light emittedby the LED chip 16 and project it toward the area to be illuminated in anarrower and more intense beam than otherwise would occur. Forchip-on-board LED array designs, reflector 26 is commonly a planarreflector made integral with conductive pad 25 by selective plating of areflective metal (such as tin solder) and is oriented radially aroundthe LED chip 16. In this case, of course then the combinedreflector/conductive pad serves the previously described functions ofboth the reflector 26 and the conductive pad 25. Suitable reflectors 26are well known to those skilled in the art and may be obtained from awide variety of optical molding and coating companies such ReedPrecision Microstructures of Santa Rosa, Calif. More than one reflector26 to be used for conventional LEDs 14 or LED chips 16 can be combinedto make a reflector array whose constituent elements are oriented insubstantial registry with the conventional LEDs 14 or LED chips 16.

As shown in FIG. 1 and FIG. 2, lens 27 is normally amagnifier/collimator which serves to collect light emitted by eachconventional LED 14 or LED chip 16 and reflected by optional reflector26 and project it in a narrower and more intense beam than otherwisewould occur. As shown in FIG. 1 for an illuminator 10 using conventionalLEDs 14, lens 27 is commonly made integral with polymer matrix enclosure18, or otherwise may be made separately from polymer matrix enclosure18. Lens 27 may also be made as an integral array of lenslets 27a whichare then substantially registered about the centers of individualconventional discrete LEDs 14.

As shown in FIG. 2 for an illuminator 10 using individual LED chips 16in a chip-on-board configuration, more than one lenslet 27a can becombined in an array to make lens 27 whose constituent elements arelenslets 27a oriented in substantial registry with the LED chips 16,reflectors 26 and pads 24 and 25. In FIG. 2, lenslets 27a are shown asTotal Internal Reflection (TIR) collimating lenses whose concave surface(facing the individual LED chips 16) consist of radial microprismstructures similar those on a Fresnel lens. However, it should beunderstood that Plano-convex, bi-convex, aspheric or their Fresnel,total-internal-reflection (TIR), catadioptric or holographic opticelement (HOE) equivalents are typical variants of lenslet 27a. Lens 27or lenslets 27a are used with a wide variety of options known to thoseskilled in the art such as color, f-number, aperture size, etc. Thesemay be obtained from various manufacturers including US precision lens,Reed Precision Microstructures, 3M, Fresnel Optics Company and PolaroidCorporation.

Referring simultaneously to FIGS. 1 and 2, one or more optionalsecondary optical elements 21 are used with the above describedconventional discrete LED designs (FIG. 1) or with LED arraydie-on-board designs (FIG. 2). Secondary optical elements 21 arecomponents that influence by combination of refraction, reflection,scattering, interference, absorption and diffraction the projected beamshape or pattern, intensity distribution, spectral distribution,orientation, divergence and other properties of the light generated bythe LEDs. Secondary optical elements 21 comprise one or more of a lens27, a deviator 28, and a diffuser 29, each of which may be inconventional form or otherwise in the form of a micro-groove Fresnelequivalent, a HOE, binary optic or TIR equivalent, or another hybridform.

A deviator 28 may be optionally mounted on or attached to the housing 19or otherwise attached to or made integral with the lens surface 27b andused to conveniently steer the collimated beam in a direction oblique tothe optic axis of the lens 27 and/or reflector 26 used in the LEDilluminator 10. Deviator 28 is normally a molded clear polycarbonate oracrylic prism operating in refractive mode for deviation angles up toabout 35 degrees or in TIR mode (such as a periscope prism) fordeviation angles in excess of 35 degrees. This prism may further bedesigned and manufactured in a microgrooved form such as a Fresnelequivalent or a TIR equivalent. Furthermore, a diffraction grating,binary optic or holographic optical element can be substituted for thisprism to act as a deviator 28. In any of these cases, the deviator 28 isconfigured as a sheet or slab to substantially cover the entire openingof the illuminator housing 19 from which light is emitted. Suchdeviators are available from the same sources as the lens manufacturerslisted above.

Optionally, a diffuser 29 may be mounted on or attached to the housing19 or otherwise attached to or made integral with the lens surface 27bor the deviator surface 28a and is used to aesthetically hide andphysically protect the illuminator internal components, and/or to filterthe spectral composition of the resultant illuminator beam, and/ornarrow, broaden or smooth the beam's intensity distribution. This can behelpful, for instance, in improving color and brightness uniformity ofthe effective illumination projected by the illuminator. Alternatively,diffuser 29 may be include unique spatial filter or directional filmsuch as Light Control Film (LCF) from 3M to sharpen the beam cut-offproperties of the illuminator 10. The diffuser 29 may furtherincorporate a unique spectral filter (such as a tinted compound or anoptical coating such as dichroic or band pass filter) to enhanceilluminator aesthetics, hide internal illuminator components fromexternal view, or correct the color of mixed light projected by theilluminator 10. Diffuser 29 is normally a compression or injectionmolded clear polycarbonate or acrylic sheet whose embossed surface orinternal structure or composition modifies impinging light byrefraction, reflection, total internal reflection, scattering,diffraction, absorption or interference. Suitable holographic diffusers29 can be obtained from Physical Optics Corporation in SouthernCalifornia, and binary optics may be obtained from Teledyne-Brown ofHuntsville, Ala.

It is preferred to have as few optical members as practical and,therefore, at least two can be combined into one integral piece. Forexample, deviator 28 can be incorporated onto an upper surface 27b oflens 27 by simply placing an appropriately machined mold insert into theplanar half of a mold for a Fresnel or TIR collimator lens. As mentionedhereinabove and shown in FIG. 2, diffuser 29 may also be attached to ormade integral with the lens surface 27b or the deviator surface 28a.Procedures for consolidating the optical members will be known to thoseskilled in the art as will substituting various individual types ofoptical members for those listed above. All such combinations areintended to be within the scope of the present invention. Clearly,whether conventional discrete LEDs 14 or individual chips 16 are used,those skilled in the art will understand that many modifications may bemade in the design of support member 12 while still staying within thescope of the present invention, and all such modifications should beunderstood to be a part of the present invention.

In accordance with the present invention, the plurality of conventionaldiscrete LEDs 14 and individual LED chips 16 consist of two types whoseemissions exhibit perceived hues or dominant wavelengths which arecolor-complementary and distinct from one another and which combine toform a metameric white light. To discuss what "metameric" and"complementary" mean in the present invention, one must understandseveral aspects of the art of producing and mixing light and the mannerin which light made from that mixing will be perceived. In general,however, it is known that the apparent "color" of light reaching anobserver depends primarily upon its spectral power distribution and uponthe visual response of the observer. Both of these must therefore beexamined.

FIG. 3 is a graph plotting the relative spectral power versus wavelengthfor Standard "white" Illuminants A, B, and C. The Standard Illuminantshave been developed by the Commission Internationale de I'Eclairage(CIE) as a reference to reduce the complexity that results from coloredobjects undergoing appreciable changes in color appearance as the lightsource which illuminates them is changed. Standard Illuminant A is ansource having the same relative spectral power distribution as aPlanckian radiator at a temperature of about 2856K. A Planckian orblackbody radiator is a body that emits radiation, because of itstemperature, according to Planck's law. True Planckian radiators areideal abstractions, not practical sources, but many incandescent sourcesemit light whose spectral composition and color bears a closeapproximation thereto. For instance, CIE Standard Illuminant A closelyapproximates the light emitted by many incandescent lamps such atungsten halogen lamp. It is convenient, therefore, to characterize thespectral power distribution of the radiation by quoting the temperatureof the Planckian radiator having approximately the same relativespectral power distribution. Standard Illuminants B and C represent"true" daylight and sunlight, respectively; however, they have toolittle power in the ultraviolet region compared with that of daylightand sunlight.

All of these Illuminants are variations of white light and, as can beseen from FIG. 3, have broadband spectral power distributions.Incandescent light sources are typically solids that emit light whentheir temperatures are above about 1000 K and the amount of powerradiated and the apparent color of this emission is directly related tothe source temperature. The most familiar incandescent light sources arethe sun, flames from a candle or gas lamp, and tungsten filament lamps.Such sources, similar to CIE Standard Illuminants A, B and C in FIG. 3,have spectral power distributions which are relatively constant over abroad band of wavelengths, are often referred to as broadband sources,and have colors which are perceived as nearly achromatic or white. Giventhe diversity of white light sources and the associated range ofnear-white colors which are de-facto accepted as white in various areasof practice, a color shall be deemed as white within the scope of thepresent invention, if it is substantially indistinguishable from or hascolor coordinates or tristimulus values approximately equal to colorswithin the white color boundary translated from the revised Kelly chart,within the SAE J578 achromatic boundaries, along the blackbody curveincluding Planckian radiators at color temperatures between 2000 K and10,000 K, sources close to Standard Illuminants A, B, C, D₆₅, and suchcommon illuminants as fluorescent F1, F2, F7, high pressure sodiumlamps, xenon lamps, metal halide lamps, kerosene lamps or candles. Theseare all well known in the art and will be referenced and discussedhereinafter.

Unlike the other sources discussed, LEDs are narrow bandwidth sources.In addition to Standard Illuminants A, B, and C, FIG. 3 shows thespectral power distribution of two LEDs, one emitting a narrow-bandwidthradiation with a peak spectral power emission at 592 nanometers (nm) andthe other at 488 nm. As can be seen by examination of this Figure, thecharacteristic spectra of LEDs is radically different from the morefamiliar broadband sources. Since LEDs generate light by means ofelectroluminescence (instead of incandescence, pyroluminescence orcathodoluminescence), the emission spectra for LEDs are determined bythe band gap of the semiconductor materials they are constructed of, andas such are very narrow-band. This narrow-band visible light emissioncharacteristic is manifested in a highly saturated appearance, which inthe present invention means they have a distinctive hue, high colorpurity, i.e., greater than about 0.8, and are therefore highly chromaticand distinctly non-white. Despite the narrow-band attributes of LEDlight, a combination of the emissions of two carefully selected LEDs cansurprisingly form illumination which appears white in color, with colorcoordinates substantially identical to Standard Illuminants A, B or C.

The reason for this is that, as mentioned hereinabove, the apparentcolor of light such as from a self-luminous source depends upon thevisual response of the observer, in addition to the characteristics ofthe light from the source. In addition, the apparent color of anon-self-luminous object or surface (one which must be illuminated by aseparate source in order to be seen) is slightly more complicated anddepends upon the visual response of the observer, the spectralreflectance of the object or surface in question, and thecharacteristics of the light illuminating the object or surface. Asillustrated in FIGS. 4a, 4b and 4c, if a surface or object is a "neutralgray" diffuse reflector, then it will reflect light having a compositionproportionally the same as the source which illuminates it, althoughinvariably dimmer. Since the relative spectral power distribution of thelight reflected from the gray surface is the same as the illuminatingsource, it will appear to have the same hue as the illuminating sourceitself. If the illuminating source is white, then the surface willappear white, gray or black (depending on its reflectance). FIG. 4cshows the resultant spectral power distribution of the light emittedfrom a plurality of amber and blue-green LEDs and subsequently reflectedfrom a 50% neutral gray target surface.

As stated hereinabove, the visual response of an observer affects theapparent color of emitted and reflected light. For humans, the sensorsor receptors in the human eye are not equally sensitive to allwavelengths of light, and different receptors are more sensitive thanothers during periods of low light levels. Cone receptors are activeduring high light levels or daylight and are responsible for colorperception. Rod receptors are active during low light levels and havelittle or no sensitivity to red colors, but have a significantsensitivity to blue light. FIG. 5 is a graph plotting the relativesensitivity of a "standard observer" versus wavelength for the spectralluminous efficiency functions. The curve represented by V represents astandard observer's visual sensitivity to stimuli seen under photopic(or high light level) conditions of vision, and the curve V' representsa standard observer's visual sensitivity to stimuli seen under scotopic(or low light level) conditions of vision. As can be seen, the photopicresponse (V) has a nearly Gaussian shape with a peak at about 555 nm andthe scotopic response (V') has a peak at about 508 nm. This differencebetween relative spectral sensitivity during photopic and scotopicconditions amounts to an enhanced blue response and diminished redresponse during darkness and is known as the Purkinje phenomenon.Scotopic conditions exist when observed surfaces have surfaceluminance's of less than a few hundredths of a candela per square meter.Photopic conditions exist when observed surfaces have surfaceluminance's of more than about 5 candelas per square meter. A transitionrange exists between Photopic and Scotopic vision and is known asMesopic (or middle light level) vision, represented by the intermediatecurve if FIG. 5 which is an estimated typical mesopic response. Anotherprimary difference between photopic, scotopic and mesopic vision is theabsence of color discrimination ability in scotopic conditions (very lowlight levels) and reduced color discrimination abilities in mesopicconditions. This will be discussed further hereinbelow.

The differences between photopic, mesopic and scotopic viewingconditions are relevant to the present invention because an illuminatoris used to illuminate areas during low light level conditions. Thusbefore any illumination, the environment represents scotopic conditionsof vision and during full illumination (after the eye has had time toadapt to the increased illumination) the environment is in the photopicconditions of vision. However, during the time the eye is adapting, andon the "outer fringes" of the illuminated region even after adaptation,the environment is in the mesopic conditions of vision. The eye'svarying sensitivities to these different levels of illumination are veryimportant in designing a proper illuminator.

The colors perceived during photopic response are basically a functionof three variables, corresponding to the three different types of conereceptors in the human eye. There are also rod receptors, however, theseonly become important in vision at low light levels and are typicallyignored in color evaluations at high light levels. Hence, it is to beexpected that the evaluation of color from spectral power data shouldrequire the use of three different spectral weighting functions. FIG. 6plots the relative response versus wavelength of the CIE color-matchingfunctions for the 1931 standard 2 degree observer. The color-matchingfunctions, x(λ), y(λ) and z(λ) relate to the sensitivity of the threetypes of cone receptors in the human eye to various wavelengths (λ) oflight through a series of transforms. As can be seen by the curves inFIG. 6, the color-matching function x(λ) has a moderate sensitivity atabout 450 nm, almost no sensitivity around 505 and a large sensitivityaround 600 nm. Another color-matching function y(λ), has a Gaussianshape centered around 555 nm, and the third color-matching function z(λ)has a significant sensitivity centered around 445 nm.

As stated earlier, it is known that by combining a red color (such as amonochromatic source located at 700 nm and hereinafter designated as R),a green color (such as a monochromatic source located at 546 nm andhereinafter designated as G) and a blue color (such as a monochromaticsource located at 435 nm and hereinafter designated as B) in properratios, virtually any color can be exactly matched. The necessaryproportions of R, G, and B needed to match a given color can bedetermined by the above described color matching functions x(λ), y(λ)and z(λ) as in the following example.

First, the amount of power per small, constant-width wavelength intervalis measured with a spectraradiometer throughout the visible spectrum forthe color to be matched. Then, the color matching functions x(λ), y(λ)and z(λ) are used as weighting functions to compute the tristimulusvalues X, Y and Z, by further using the following equations:

    X=k P.sub.1 x(λ).sub.1 +P.sub.2 x(λ).sub.2 +P.sub.3 x(λ).sub.3 +. . . P.sub.n x(λ).sub.n          1!

    Y=k P.sub.1 y(λ).sub.1 +P.sub.2 y(λ).sub.2 +P.sub.3 y(λ).sub.3 +. . . P.sub.n y(λ).sub.n          2!

    Z=k P.sub.1 z(λ).sub.1 +P.sub.2 z(λ).sub.2 +P.sub.3 z(λ).sub.3 +. . . P.sub.n z(λ).sub.n          3!

where k is a constant; P₁,2,3,n are the amounts of power per smallconstant width wavelength interval throughout the visible spectrum forthe color to be matched and x(λ)₁,2,3,n, y(λ)₁,2,3,n and z(λ)₁,2,3,n arethe magnitudes of the color-matching functions (taken from the curves ofFIG. 6) at the central wavelength of each interval. Finally, theapproximate desired proportions of the above described monochromaticsources R, G and B are calculated from the above computed X, Y and Ztristimulus values using the following equations:

    R=2.365X-0.897Y-0.468Z                                      4!

    G=-0.515X+1.426Y+0.0888Z                                    5!

    B=0.005203X-0.0144Y+1.009Z                                  6!

Therefore, the color-matching functions of FIG. 6 can be used asweighting functions to determine the amounts of R (red), G (green) and B(blue) needed to match any color if the amount of power per smallconstant-width interval is known for that color throughout the spectrum.Practically speaking, R, G and B give the radiant intensities of 3monochromatic light sources (such as lasers) with emissions at 700 nm,546 nm and 435 nm, respectively, needed to match the chosen color.

Referring again to FIG. 3, the reason that the combined emissions fromthe two depicted LEDs will look like a broadband white light source,even though they possess radically different spectral compositions, isbecause their combined emissions possess the same tristimulus values (ascomputed by Equations 1-3) as those of the broadband source StandardIlluminant B. This phenomenon is known as metamerism and is an essentialaspect of the present invention.

Metamerism refers to a facet of color vision whereby two light sourcesor illuminated objects may have entirely different emitted or reflectedspectral power distributions and yet possess identical tristimulusvalues and color coordinates. A result of metamerism is that additivemixtures of light from two completely different pairs of luminoussources (with their associated distinct spectra) can produceillumination having exactly the same perceived color. The principles andapplication of additive color mixing and metamerism to the presentinvention are discussed in greater detail later in this disclosure.

FIG. 7 is a CIE 1976 uniform chromaticity scale (UCS) diagram, commonlyreferred to as the u', v' diagram. The u', v' diagram is used toconveniently provide numerical coordinates that correlate approximatelywith the perceptible color attributes, hue, and saturation. A UCSDiagram is also used to portray the results of color matchingcomputations, color mixing and metamerism in a form that is wellrecognized in the art and is relatively easy to understand and use. Ofcourse, exact color perceptions will depend on the viewing conditionsand upon adaptation and other characteristics of the observer. Inaddition, other color coordinate systems are available, such as the CIE1931 2 degree Chromaticity Diagram (commonly referred to as the x, ychart), CIELAB, CIELUV, Hunter and Munsell systems to name a few. Forthe sake of simplicity, the present invention is further describedhereinbelow using the CIE 1976 UCS system. However, it should beunderstood that teachings of the present invention apply regardless ofthe color system used to describe the invention and therefore are notlimited by this exclusive use of the CIE 1976 UCS system.

Referring again to FIG. 7, the location of a color on the u', v' diagramis obtained by plotting v' and u', where:

    u'=4X/(X+15Y+3Z)=4x/(-2x+12y+3)                             7!

    v'=9Y/(X+15Y+3Z)=9y/(-2x+12y+3)                             8!

and where X, Y and Z are the tristimulus values described hereinabove (xand y correspond to the CIE 1931 Chromaticity x, y coordinates and areprovided for convenient conversion). Thus, any color can be described interms of its u' and v' values. FIG. 7 shows the respective positions onthe u', v' diagram for the Planckian Locus, the SAE J578 boundaries forachromatic white light, Standard Illuminants A, B and C, and well as thelocus of binary additive mixtures from blue and red LEDs are shown. Ascan be seen, Standard Illuminants A, B, and C, closely corresponding toblackbody radiators, lie along the Planckian Locus.

The Planckian Locus is a curve on the u', v' diagram connecting thecolors of the Planckian radiators at various temperatures, a largeportion of which traverses the white, or achromatic region of thediagram. The SAE J578 achromatic white boundaries shown were translatedfrom CIE 1931 Chromaticity x, y coordinates using Equations 6 and 7hereinabove and are generally used to define what is an acceptable whitelight for automotive purposes (although many automotive white lights inuse fall outside these boundaries).

Also shown in FIG. 7 is the range of colors produceable by ahypothetical additive color-combination of red (660 nm) and blue (460nm) LEDs. FIG. 7 clearly shows how far off the Planckian Locus and theSAE J578 achromatic boundaries that the colors produced by thiscombination fall. In fact, the locus of binary additive mixtures fromthese blue and red LEDs has perceived hues of red, pink, purple, violetand blue. This system would therefore not be suitable as the improvedwhite-light illuminator of the present invention.

A white light illuminator might in fact be constructed, however, from athree-color system. As stated hereinabove, a R-G-B combination canproduce almost every conceivable color on the 1976 UCS Diagram. Such asystem would be complex and expensive, however, and/or would suffer fromunacceptable manufacturing variations inherent to R-G-B systems.

This is illustrated best by reference to FIG. 8, which again shows theCIE 1976 UCS Diagram, the Planckian Locus and the translated SAE J578boundaries. In addition, the locus of ternary additive mixturesproduceable from hypothetical R-G-B LED configuration combinations andestimated manufacturing variations associated therewith are shown. Dueto various uncontrolled processes in their manufacture, LEDs of anygiven type or color (including red, green and blue) exhibit largevariations from device to device in terms of their radiant and luminousintensities, and smaller variations in hue. As can be confirmed byreference to typical LED product literature, this variation canrepresent a 200 percent change in intensity from one LED to another orfrom one batch to another, even if the compared LEDs are of the sametype and hue. Equations 7 and 8 clearly show the dependency of a color'su', v' coordinates upon its tristimulus values X, Y and Z, and equations4-6 show a linking dependency to source power (or intensity). Thus,variations in R-G-B LED intensity and hue will cause variations in theu', v' color coordinates of their mixed light. Therefore it would bevery difficult to construct a large number of R-G-B LED illuminatorswith any assurance that their light would reproducibly match a desiredcolor such as a white. This is illustrated by the shaded area in FIG. 8and is referred to as the Locus of R-G-B LED Manufacturing Variation.

Thus, with a red, green, blue (RGB) combination, white light can bereproducibly created only if extraordinary measures were invented toensure that the additive color mix proportions are maintained during LEDand illuminator assembly production. This would involve extensivemeasurement and for every LED to be used or perhaps incorporation ofactive electronic control circuits to balance the LED output in responseto some process sensor. The extra costs and complexity associated withsuch an approach, combined with the obvious complexity of supplyingthree different types of LEDs through inventory and handling systems isdaunting and renders such a configuration unsuitable for the illuminatorapplications of the present invention.

In the broadest sense, therefore, the present invention relates toproducing nearly achromatic light by additively combining complementarycolors from two types of colors of saturated LED sources or theirequivalents. By complementary we mean two colors that, when additivelycombined, reproduce the tristimulus values of a specified nearlyachromatic stimulus, e.g. a reference white. By appropriately tailoringthe proportions of light from each of these two complementary colors, weproduce a metameric white resultant color, or alternatively any otherresultant color between the two complementary color stimuli (dependingon the proportion of the additive mixture). Although the saturatedsources of greatest interest are LEDs, whose emissions are narrow-band,the present invention clearly teaches that similar results could beachieved with other appropriately chosen narrow-band light sources.

FIG. 9 is a CE 1976 UCS diagram which broadly illustrates how theadditive mixture of light from two LEDs having complementary hues can becombined to form a metameric white light. Also shown are the approximateboundaries of the "white" color region which has been translated fromthe revised Kelly chart and the CIE 1931 x, y Chromaticity Diagram. TheKelly chart and 1931 x, y Chromaticity Diagram are not shown but arewell known in the art. FIG. 9 further depicts a first embodiment of thepresent invention utilizing a combination of one or more LEDs whoseemissions have peak wavelengths of approximately 650 nm and 500 nm andperceived hues of red and green, As the diagram shows, this produces a"white" light located between Standard Illuminants A and B on thePlanckian Locus.

It should be understood, however, from the above discussions that,substantial variations inherent to conventional discrete and individualchip LEDs will cause changes in the coordinates of the resultantadditive color mixture. The 650 nm LED depicted in FIG. 9 may fall intoa range of LEDs with peak wavelengths ranging from 635 to 680 nm whoselight has the hue of red, and the 500 nm LED depicted in FIG. 9 may fallinto a range of LEDs with peak wavelengths ranging from 492 nm and 530nm and whose light has the hue of green. In this embodiment, thisvariation, and more particularly the pronounced intensity manufacturingvariations of the plurality of LEDs used, will cause the coordinates ofthe resultant mixture to traverse the u', v' chart in a directiongenerally substantially perpendicular to the Planckian Locus into eitherthe yellowish pink or the yellowish green region of the u', v' diagram.Fortunately, as discussed hereinabove, there is some tolerance in thehuman visual system for acceptance of slightly non-white colors aseffectively white. It should be understood that a similar mixture ofred-orange or red LED light (with a peak wavelength between 600 nm and635 nm or between 635 and 680 nm, respectively) with a complementarygreen LED light (with a peak wavelength between 492 nm and 530 nm) or amixture of yellow-green or yellow LED light (with a peak wavelengthbetween 530 nm and 572 nm) with a purple-blue or blue LED light (with apeak wavelength between 420 nm and 476 nm) can be made to function inthe same manner to produce similar results and are included in the scopeof this embodiment of the present invention. Thus a system as describedherein would function as an embodiment of the present invention if theother parameters were also met (such as projecting effectiveilluminance).

A more preferred embodiment is illustrated in FIG. 10 which is a CIE1976 UCS diagram illustrating a binary complementary combination oflight from a plurality of LEDs of two different types having peakwavelengths of 592 nm and 488 nm and perceived hues of amber andblue-green, respectively, such that the light from the two types of LEDsoverlaps and mixes with sufficient intensity and appropriate proportionto be an effective illuminator projecting white light. Although theirspectra are very different from that of any Standard Illuminant, themixed output of an amber LED and a blue-green LED appears surprisinglyto be almost identical to Standard Illuminant B or C when viewed by a"standard" human observer. On FIG. 10, the u', v' coordinates of thenominal mixture occur at the intersection of this dashed line and thePlanckian Locus, between Standard Illuminants A and B. Since the u', v'coordinates of the LED colors in this embodiment mark the endpoints of aline segment which is substantially coaxial with the Planckian Locus andthe long axis of the SAE J578 achromatic white boundaries, any intensityvariation deriving from manufacturing variations will produce colorsalong an axis remaining in close proximity to the Planckian Locus andwithin the boundaries of other widely-accepted definitions of white.This significantly simplifies the manufacturing process and controlelectronics associated with the illuminator, which decreases the overallproduction cost and makes commercialization more practical. In addition,we have found that of the many types and hues of LEDs presentlyavailable, the two preferred types of LEDs for the present inventionhave very high luminous efficacy in terms of light emitted compared toelectrical power consumed. These are transparent substrate AlInGaP amberLEDs available from Hewlett Packard Inc., Optoelectronics Divisionlocated in San Jose, Calif. and GaN blue-green LEDs available fromNichia Chemical Industries, LTD located in Anan-shi, Tokushima-ken,Japan.

FIG. 11 further amplifies this embodiment of the invention byillustrating issues of manufacturing variation within the context ofother definitions of white. The hatched lines between amber (peakemission between 572 nm and 600 nm) and blue-green (peak emissionbetween 476 nm and 492 nm) show the range in LED hue variations ateither endpoint for this embodiment which would be generally capable ofproducing metameric white light. Since LEDs are solid-state devicescomprising a base semiconductor material and one or more dopants whichimpact the spectral emission characteristics of the LED, the level ofdoping and other process parameters can be adjusted to intentionallymodify the peak wavelength emitted by the LED. Furthermore, as discussedhereinabove, certain random variations also occur, affecting theadditive color mixture. In this embodiment of the present invention,however, larger than normal variations can be tolerated. This is becausea large part of the area between the hatched lines and within themonochromatic locus of the chart overlaps the areas commonly perceivedas and referred to as white, such as the Planckian Locus, the markedregion corresponding to the translated Kelly boundaries for the colorwhite, or the shaded region corresponding to the translated SAE J578boundaries for achromatic white. Therefore, all of the additive colorsresulting from reasonable variations in the LED intensity and hue ofthis embodiment fall within one of the white regions. The Figure thusclearly illustrates how there can be a range of amber LEDs whose huesare complementary with a range of blue-green LED hues which, whencombined, form substantially white light.

The most preferred embodiment of the present invention uses a binarycomplementary combination of light from a plurality of LEDs of twodifferent types having peak wavelengths of 584 nm and 483 nm andperceived hues of amber and blue-green respectively, such that the lightfrom the two types of LEDs overlaps and mixes with sufficient intensityand appropriate proportion to project effective white illumination. Whenplotted on a color chart, the u', v' coordinates of the light emitted bythe LEDs of this embodiment mark the endpoints of interconnecting linesegment which is coaxial with the portion of the Planckian Locus whichtraverses Standard Illuminants A, B and C, as shown in FIG. 12.

As discussed hereinabove, intensity and hue variations are a naturalby-product of random variations occurring in production of LEDs. Forthis embodiment of the present invention, however, the need forintensive in-process manufacturing controls and electronic controlsintegrated onto an illuminator assembly to compensate for inherentmanufacturing variations for LEDs is largely eliminated. This isillustrated by the substantially coaxial relationship between the lineconnecting the u' v' coordinates of the preferred LEDs of the presentinvention and a best-fit linear approximation to the portion of thePlanckian Locus from 2000 K to 10000K. In addition, process controls,inventory management, materials handling, and electronic circuit designare further simplified by only having two colors to manipulate ratherthan three. This substantial simplification decreases manufacturingcosts significantly and augments the present invention's capability forcreating and projecting white light--the only color of light desired forthe practical embodiments of the present invention.

The flexibility of the present invention is further amplified byapplication of additive color techniques to synthesize the end memberconstituents of the binary-complementary LED light mixture describedhereinabove. This approach is best understood by reference to FIG. 13,which illustrates the use of additive subcombinations ofnon-complementary LEDs to form effective binary complementscorresponding to the two types of LEDs discussed hereinabove. Hues 1-7represent the emissions from LEDs as follows: hue 1 is purple-blue orblue for LEDs with a peak wavelength between 420 nm and 476 nm, hue 2 isblue-green for LEDs with a peak wavelength between 476 nm and 492 nm,hue 3 is green for LEDs with a peak wavelength between 492 nm and 530nm, hue 4 is yellow-green or yellow for LEDs with a peak wavelengthbetween 530 nm and 572 nm, hue 5 is amber for LEDs with a peakwavelength between 572 nm and 605 nm, hue 6 is red-orange for LEDs witha peak wavelength between 605 nm and 635 nm and hue 7 is red for LEDswith a peak wavelength between 635 nm and 680 nm. An additive mixture oflight from one or more LEDs with hues 6 or 7 and one or more LEDs withhue 4 can be combined to form light having the same hue andsubstantially the same saturation as LED light with hue 5. Thus anequivalent or substitute for the amber LEDs of FIG. 11 is synthesized byadditive combination of the emitted light from two types of LEDs whoseemissions are characterized by hues 6 or 7 and 5, respectively. In asimilar fashion, an additive mixture of light from one or more LEDs withhue 1 and one or more LEDs with hue 3 can be combined to form lighthaving the same hue and substantially the same saturation as LED lightwith hue 2, thus synthesizing an equivalent or substitute for theblue-green LEDs of FIG. 11.

When a non-complementary sub-combination of LED light is used tosynthesize an equivalent or substitute to one of the end members of theaforementioned binary complementary mixture, then the resultant lightfrom the sub-combination is mixed with its binary complement oreffective binary complement and projected via a lens and/or otheroptical elements form an effective metameric white illumination. Thiscan be important in commercial practice, where prolonged supplydisruptions are common for LEDs of one variety or another due toexplosive growth in market demand or insufficient LED manufacturercapacity. As explained herein, such a disruption can be mitigated in thecase of the present invention by the use of sub-combinations of morereadily available alternative LEDs to form equivalent complements.

FIGS. 14a-c illustrate an illuminator of the present inventionincorporated as a maplight within an automotive interior rearviewmirror, however it should be understood that the illuminator of thepresent invention may alternatively be incorporated into an automotiveexterior rearview mirror as a security light or "puddle" light. Theautomotive rearview mirror 130 is provided with a housing 132 composedof a back wall 132a, a peripheral sidewall 132b having a top, bottom andendwall portion. The peripheral sidewall 132b defines a front openingadapted to receive a mirror element 134. A mounting bracket (not shown)may be provided for mounting the rearview mirror 130 on an automobilewindshield (not shown) or headliner (not shown). The mirror element 134may be a conventional prismatic mirror element as shown in FIGS. 14a and14b or may be an electro-optic glare reducing mirror element such as anelectrochromic or liquid crystal dimming mirror element well known inthe art. It should be understood that, although FIG. 14 shows aconventional prismatic mirror element, the mirror element 134 isintended to represent any mirror element well known in the art includingan electrochromic glare reducing mirror element within deviating fromthe scope the present invention.

If an electrochromic glare reducing mirror element is substituted asmirror element 134, the following list of patents provides an exemplaryteaching of electro-optic devices in general and, more specifically,electrochromic rearview mirrors and associated circuitry. U.S. Pat. No.4,902,108, entitled "Single-Compartment, Self-Erasing, Solution-PhaseElectro-optic Devices Solutions for Use Therein, and Uses Thereof",issued Feb. 20, 1990 to H. J. Byker; Canadian Patent No. 1,300,945,entitled "Automatic Rearview Mirror System for Automotive Vehicles",issued May 5, 1992 to J. H. Bechtel et al.; U.S. Pat. No. 5,128,799,entitled "Variable Reflectance Motor Vehicle Mirror", issued Jul. 7,1992 to H. J. Byker; U.S. Pat. No. 5,202,787, entitled "Electro-OpticDevice", issued Apr. 13, 1993 to H. J. Byker et al.; U.S. Pat. No.5,204,778, entitled "Control System For Automatic Rearview Mirrors",issued Apr. 20, 1993 to J. H. Bechtel; U.S. Pat. No. 5,278,693, entitled"Tinted Solution-Phase Electrochromic Mirrors", issued Jan. 11, 1994 toD. A. Theiste et al.; U.S. Pat. No. 5,280,380, entitled "UV-StabilizedCompositions and Methods", issued Jan. 18, 1994 to H. J. Byker; U.S.Pat. No. 5,282,077, entitled "Variable Reflectance Mirror", issued Jan.25, 1994 to H. J. Byker; U.S. Pat. No. 5,282,077, entitled "VariableReflectance Mirror", issued Jan. 25, 1994 to H. J. Byker; U.S. Pat. No.5,294,376, entitled "Bipyridinium Salt Solutions", issued Mar. 15, 1994to H. J. Byker; U.S. Pat. No. 5,336,448, entitled "ElectrochromicDevices with Bipyridinium Salt Solutions", issued Aug. 9, 1994 to H. J.Byker; U.S. Pat. No. 5,434,407, entitled "Automatic Rearview MirrorIncorporating Light Pipe", issued Jan. 18, 1995 to F. T. Bauer et al.;U.S. Pat. No. 5,448,397, entitled "Outside Automatic Rearview Mirror forAutomotive Vehicles", issued Sep. 5, 1995 to W. L. Tonar; and U.S. Pat.No. 5,451,822, entitled "Electronic Control System", issued Sep. 19,1995 to J. H. Bechtel et al. Each of these patents is commonly assignedwith the present invention and the disclosures of each, including thereferences contained therein, are hereby incorporated herein in theirentirety by reference.

In accordance with one embodiment of the present invention, the bottomportion of the housing peripheral sidewall 132b of mirror 130 has twoopenings 140a and 140b disposed therein such that a portion of thevehicle is illuminated therethrough. Two sets of a plurality of LEDs 114are disposed within the housing 130 such that when energized by a powersupply (not shown) and/or electronic control 122 a portion of thevehicle interior is illuminated through openings 140a and 140b. As isshow in FIG. 14a, openings 140a and 140b are disposed toward oppositeends of the bottom portion of peripheral sidewall 132b such that opening140a illuminates the driver-portion of the vehicle interior and opening140b illuminates the passenger-portion of the vehicle. For mirrorsdesigned for certain foreign vehicles having a right-hand driveconfiguration, of course, opening 140a would correspond to thepassenger-portion of the vehicle and opening 140b would correspond tothe driver-portion.

Incorporating LEDs 114 into housing 132 to illuminate the vehicleinterior through openings 140a and 140b has a number of advantages overprior art incandescent maplights as follows.

Incandescent illuminators operate by heating a metal filament and asignificant portion of this heat radiates, conducts and convects awayfrom the bulb. This heat must be dissipated to reduce the chance ofdamage to the mirror assembly or other components within the mirrorhousing, e.g., electrochromic glare reducing element, compass, etc.Mechanisms for dissipating this heat are features such as heat sinkswhich assist with conductive and convective heat transfer, air vents orblowers which improve convection, or certain optical components orcoatings which can help with radiative heat transfer. All of thesetypically which add a disadvantageous combination of weight, volume,cost or complexity to the mirror assembly.

In addition, incandescent lights radiate light equally in alldirections. This causes several problems such as having to incorporatelarge reflectors to direct the light toward the vehicle occupants. Thesereflectors in-turn occupy critical space and add weight to the mirrorassembly. Furthermore, light from the incandescent source that is notreflected by the reflector toward the occupants can cause glare to thedriver and can also inhibit the proper operation of any electrochromicglare reducing layer by causing a false input to the glare and/orambient light sensors incorporated therein. LEDs, whether theconventional discrete type with integral optics or individualsemiconductor die with separate optics, are very small and therefore thereflector assemblies or other optics used with them do not addsignificantly to the weight or volume of the mirror assembly. Inaddition, several LED chips may be incorporated into one package foreven further reduction is size.

As weight is added to the mirror assembly, greater stress is placed uponits mounting structures and its resonant vibration characteristicschange. Greater stress on the mounting mechanism due to any increase inweight can lead to premature failure of the mounting mechanism,particularly if the mount is of the type which attaches by means ofadhesive to the interior surface of the windshield. The added weight canalso cause a decrease in resonant frequency and increase in vibrationalamplitude, degrading the clarity of images reflected in the mirrorduring vehicle operation. In addition to being a safety concern,premature mount failure or increased vibration signature would clearlywould be displeasing to the vehicle owner.

As automobiles become more complex, more and more option-components arebeing incorporated in the mirror housing. For example, remote keylessentry systems, compasses, indicia for direction, tire pressure,temperature, etc., are being incorporated into mirror housings. Sincethere is limited space in a mirror housing, decreasing the volume ofevery component within the housing is necessary. The additional spacerequired to cool an incandescent lamp and collimate its light severelycomplicates the inclusion of these other desirable features.

Conversely, LEDs do not operate at high temperatures and thus createfewer heat dissipation problems and the space problems associated withheat dissipation measures.

Because individual LED chips are extremely small, typically measuring0.008 inches×0.008 inches×0.008 inches, they approximate a point sourcebetter than most incandescent filaments and the collimating optics (suchas lenses and reflectors) used with either the conventional discreteLEDs or chip-on-board LEDs can perform their intended function with muchgreater effectiveness. The resultant LED illuminator projects a moreuniform and precisely tailored and directed intensity distribution.

LEDs have an extraordinarily long life compared with the typical1,000-2,000 hour life for incandescent lights. A typical LED will lastanywhere from 200,000 to 2 million hours, depending on its design,manufacturing process, and operating regime. LEDs are also much morerugged than incandescent bulbs; they are more resistant to mechanicalshock and vibration, thermal shock and impact from flying debris. Theyfurther are virtually impervious to the on-off switching transientswhich cause substantial reliability problems for incandescent systems.The lifetime and reliability advantage is significant, and when coupledwith their inherent ruggedness, the advantage of using LEDs becomesstriking.

Comparing an amber LED (Part No. HLMT-DL00, from Hewlett Packard) with a0.72 W power dissipation from the circuit in FIG. 21, with a Philipstype 192 lamp run at 13.0 Volt, using the method set forth in MilitarySpecification HDBK-217F-1, illustrates the significant disparity incalculated failure rate. The results show that the amber LED would havea 0.17 percent failure rate whereas the incandescent lamp would have afailure rate of 99.83 percent over the same time period.

In exterior rearview mirrors these issues are further amplified, due tothe more severe shock and vibration conditions, as well as environmentalexposures such due to rain, snow, temperature fluctuations, UV radiationexposure and humidity which prevail in the outdoor environment. Thismakes incorporating an incandescent lamp into an outside rearview mirroreven more difficult in that they must be protected from these factors.Regardless of the measures undertaken to prevent failure of incandescentlamps incorporated into an automotive interior mirror map light assemblyor into an automotive exterior mirror security light assembly, theselamps have such a short life that means must be provided for replacingthe light bulbs without having to replace the entire mirror assembly.Unfortunately, a design which allows for easy replacement typically isnot as effective at protection, further increasing the probability ofearly failure. This makes the task of protecting the bulb fromenvironmental factors difficult and costly to design and manufacture.LEDs, on the other hand, have an extremely long life and are generallyvery highly resistant to damage from vibration, shock and otherenvironmental influences. Therefore, LEDs last much longer than the lifeof the mirror assembly and the vehicle itself, and the design of themirror assembly need not include means for replacing the LEDs.

White-light LED illuminators of the present invention can be verycompact and thus can be incorporated into automotive rearview mirrors ina manner with much greater aesthetic appeal than with prior-artincandescent systems.

Finally, incandescent lamps possess very low electrical resistance whenfirst energized until they heat up to incandescence and therefore drawan in-rush current which can be 12-20 times their normal operatingcurrent. This inrush places tremendous thermo-mechanical stresses on thefilament of the lamp, and commonly contributes to premature failure at afraction of rated service life (much shorter than the service life of avehicle). Inrush current also stresses other electronic components in orattached to the illuminator system such as the power supplies,connectors, wire harnesses, fuses and relays such that all of thesecomponents must be designed to withstand repeated exposure to this largetransient. LEDs have no in-rush current and therefore avoid all of theseissues.

The "bloom time" for incandescent lamp or time it takes for the lamp tobecome fully bright after its supply voltage is initially applied isvery long-in excess of 0.2 seconds for many lamps. Although extremelyfast response times are not mandatory in a vehicle maplight, a fastresponse characteristic is advantageous for electronic control ofintensity and color mix proportions as discussed below. Further, certainbinary complementary metameric white LED illuminator applications suchas lamps aiding surveillance may benefit from a strobe-like ability tobecome bright quickly upon electronic activation.

In accordance with one embodiment of the present invention the pluralityof LEDs 114 behind each opening 140a and 140b may be any combination oftwo types of LEDs whose emissions have hues which are complements to oneanother (or a combination of equivalent binary complements formed fromnon-complementary sub-combinations of LED light) such that the lightfrom the two groups combines to project effective white illumination;however, as stated above, the preferred LEDs 114 disposed in each of theopenings 140a and 140b are a combination of amber LEDs and blue-greenLEDs. When energized, these two types of LEDs produce light having hueswhich are color compliments, and, when combined in the properproportions, their resultant beams overlap and mix with sufficientintensity to be an effective illuminator projecting substantially whitelight. More specifically, at least two amber LEDs such as a transparentsubstrate AlInGaP type from Hewlett Packard Corporation, OptoelectronicsDivision should be combined with at least one blue-green LED such as aGaN type LED from Nichia Chemical Industries, LTD. in each of openings140a and 140b; the most preferred combination is 3 or 4 amber LEDs to 2or 3 blue-green in each of openings 140a and 140b. This combinationproduces white light with an effective illumination to illuminate aportion of a vehicle's interior to assist the occupants in reading maps,books, and the like.

As stated above, an area of effective illumination occurs at somedistance away from the illuminator. Effective illumination is animportant aspect of the present invention and, in the art of automotivemaplights, is partly determined by auto manufacturer specifications. Forexample, FIG. 15 indicates what one auto manufacturer requires as anacceptable illuminance for a rearview mirror having an integral maplamp. The illuminance measurements must be recorded for the drivers sideat points 1-13. The average illuminance at points 1-5 must be no lessthan 80 lux with the minimum measurement of these points no less than 13lux; the average illuminance at points 6-9 must be no less than 30 luxwith the minimum measurement of these points no less than 11.5 lux; andthe average intensity at points 10-13 must be no greater than 30 lux (toavoid glare). The illuminance measurements must be recorded for thepassengers side at points 14-26 and the average illuminance at points14-18 must be no less than 80 lux with the minimum measurement of thesepoints no less than 13 lux; the average illuminance at points 19-22 mustbe no less than 30 lux with the minimum measurement of these points noless than 11.5 lux; and the average illuminance at points 23-26 must begreater than 30 lux.

FIG. 16 illustrates schematically how the illuminator 10 of the presentinvention meets the above specifications. A section view is shown of anilluminator 10 similar to that of FIG. 1, but with five conventionaldiscrete T 13/4 LEDs 14 (three amber and two blue-green) illuminating atarget surface at a distance R1, which is approximately 22 inches for anautomotive interior mirror maplight. The points labeled T1-T7 representreference points on a target at which minimum and/or maximum illuminancerequirements are typically specified. The Figure also illustratesoverlapping beams from a plurality of two different types of LEDs whichare emitting light having complementary hues, e.g. blue-green and amber.The beams of the plurality mix as they travel outward from the LEDs,overlapping to give a greater illuminance and form abinary-complementary additive color mixture of metameric white light. Itshould be understood that, for some uses of the illuminator of thepresent invention, such as a pocket flashlight, it is sufficient to usea plurality consisting of a single amber LED and a single blue-green LEDof the types described above. Of course, other applications of theilluminator, such as an electric bicycle headlamp, require many more ofeach type of LED in order to meet industry and regulatoryspecifications.

An important criterion for an effective illuminator is that itsprojected light must conform to accepted definitions of white light aspreviously described at reasonable operating ranges. Inasmuch as theadditive complementary color mixture of the present invention depends onoverlapping of projected beams from the member LEDs of the plurality inthe illuminator, it is important to understand that each illuminator ofthe present invention will have a minimum operating distance forwell-blended metameric white light. Depending on the actual LED arrayand associated optics utilized in a given embodiment, this distance willvary widely. Typically, good beam mixing (and thus balanced additivecomplementary light combination to produce reasonably uniform whitelight) requires a minimum operating range of about 10 times the averagedistance between each LED and its nearest color complement in theplurality. This minimum operating range for good beam mixing is verydependent on the application requirements and optics used, however, andcan be a much larger multiple of complementary LED pitch spacing. For anautomotive map light illuminator incorporated of the present inventionas illustrated in FIGS. 14 and 16, into an interior rearview mirrorassembly, a typical dimension between complementary conventionaldiscrete T 13/4 LEDs in the plurality is about 0.4 inches and theminimum distance for reasonably uniform white is illumination is about12 inches. Since the specified target for this embodiment is 22 inchesaway, this minimum operating range for uniform white illuminationpresents no problem. It should be noted that the illuminator of thepresent illumination does project illumination at ranges shorter thanthis specified range (as well as longer). The color and illuminancelevel of the projected light is typically not as uniform at shorterranges, however.

The pitch spacing between LEDs, array size of the plurality of the LEDsin the illuminator and the characteristics of the collimating optics anddiffusers used determine the distribution of constituent light in theilluminator's beam. Fortunately, these can be tailored to meet almostany desired combination of far-field intensity distribution, aperture,beam cut-off angle, color uniformity, and minimum operating range foreffective uniform white illumination. For an electric bicycle headlamp,the predetermined distance for effective white illumination may be 5feet and conventional discrete LEDs may be suitable as the plurality inthe illuminator. For an instrument panel indicia backlight, however, thepredetermined distance for effective uniform illumination may be 0.25inches or less and a chip-on-board LED array using low f# lenslets willalmost certainly be required.

Referring again to FIG. 16, the level of mixing of light from the 5LEDs, as well as the luminous output, depend on the distance R1 and alsodepend on the distance D between complementary LEDs. If the LEDs in theplurality are packed closer together, the light mixes completely at ashorter projected distance and the uniformity of the color andilluminance of the combined beam is improved. The pitch spacing Dbetween complementary LEDs in the plurality can vary widely fromapproximately 0.020 inches (for a chip-on-board LED array) to as much as3 inches for a spotlight or greater for various floodlight applications,but is preferably as small as possible. Conventional discrete LEDs oftenhave their own integral optical elements assembly and therefore there isa limit on how closely they can be packed together. The five T 13/4 LEDsused to gather the above data were placed in a row approximately 0.4inches apart.

Irrespective of whether conventional discrete LEDs or individual die areused, an optical element should be incorporated into the illuminatorassembly to direct the generated light toward the desired surface andinfluence the distribution of the intensity generated by the LEDsthrough the use of one or more of a lens, a diffuser, a reflector, arefractor, a holographic film, etc. as discussed hereinabove.

For the automotive maplight illuminator of FIGS. 14 through 20, twoblue-green GaN T 13/4 LEDs from Nichia were operated at 24.5 milliampsand the 3 amber TS AlInGaP T 13/4 LEDs from Hewlett Packard wereoperated at about 35 milliamps. A 10 degree embossed holographic LightShaping Diffuser (LSD) from Physical Optics Corporation was used tosmooth and distribute the illuminator beam.

FIG. 17 shows a perspective three dimensional representation of theintensity distribution from an automotive interior maplight illuminatorembodiment of the present invention. The Gaussian aspect of this plottedsurface shows an important benefit of the present invention--that theintensity distribution of the illuminator is easily crafted to be asmoothly-varying, monotonic function with respect to angular deflectionfrom the primary optical axis of the illuminator. In contrast to this,many prior-art illuminators are prone to intensity irregularities whichcan cause localized visibility distortions in the target area to beilluminated. FIG. 18 is a two dimensional contour plot amplifying theintensity information given in FIG. 17 for the same illuminator.

In order for an LED illuminator to be effective, the projected beamsfrom the plurality of LEDs must overlap one another, such that, asdiscussed hereinbefore, a complementary color mixture occurs to producemetameric white light. In addition, the illuminator must projectsufficient intensity in a desired direction to illuminate objects orsurfaces at that distance to a light level where useful visualdiscrimination is possible, even in low ambient light conditions. Usefulvisual discrimination requires color contrast and luminance contrastbetween separate objects or images and this demands enough light forcolor vision to occur, that is photopic or mesopic conditions. Photopicvision occurs when viewing objects or surfaces which have a surfaceluminance greater than approximately 5 candelas per square meter (5nit), whereas Mesopic vision can reasonably be expected when viewingobjects or surfaces which have a surface luminance greater thanapproximately 0.5 candela per square meter (0.5 nit). For surfaces whichare neutral gray, Lambertian and have a reflectance factor of 50% ormore, Photopic and Mesopic levels of surface luminance will thereforeoccur with illuminances of approximately 30 lumens per square meter (30lux) and 3 lumens per square meter (3 lux), respectively. For anilluminator 1 meter removed from this surface, the required intensityfor Photopic and Mesopic vision are therefore 30 candelas and 3candelas, respectively. The relationships between intensity,illuminance, and luminance are welt known in the art and will not bediscussed in further detail herein.

FIG. 19 shows the measured projected illumination pattern from theilluminator of FIGS. 17 and 18. The data shown were taken with acosine-corrected illuminance meter from a target whose center distancefrom the illuminator was 22 inches. The values shown, as for those inFIGS. 17, 18 and 20 represent initial values taken within approximately30 seconds of initial power-on. A comparison of FIG. 19 and FIG. 15shows that the illuminator of the present invention meets or exceeds therequirements of an auto manufacturer for an automotive interior mirrormap light. Note that the illumination level in the outer target zonewhich is required to be less than 30 lux by the manufacturer is actuallyonly about 7 lux in the case of the present invention. This isaccomplished without compromising the other minimum illuminancerequirements (such as in the target center which must be greater 80 lux)and illustrates the superior directional control achieved in the presentinvention. This provides a significant safety advantage in that thelight which is most distracting to the driver is much less with the LEDilluminator of the present invention than with a prior art incandescentlight. This advantage is also applicable to vanity mirrors, readinglamps and dome lights because the illuminator can be directed to whereit is wanted and very little illumination goes where it is not wanted.In summary, the LED illuminators of the present invention are moreeffective at placing light where it is desired and keeping it away fromareas where it is undesirable.

FIG. 20 shows a simplified luminance map of a target surface with ahypothetical neutral gray Lambertian reflectance of 50%. Note the largearea within which Photopic levels of surface luminance are maintained.In the present invention, this zone also coincides with theminimum-sized zone within which the projected illumination possesses ametameric white color as previously defined. Thus, maximum colorcontrast and luminance contrast are made to coincide in the mostcritical central portion of the target area, giving observers the bestvisual discrimination possible.

The inventors have discovered that, outside of this effective Photopicwhite illumination zone (corresponding to Photopic levels of luminancefor a 50% neutral gray target), substantial economy may be achieved byallowing the color of the additive mix to stray somewhat from accepteddefinitions of white. Surprisingly, outside of this zone the color ofthe light from the illuminator isn't as tangible to the unaided eye.This is apparently because the capability for human vision to perceivecolors falls off rapidly as surface luminance falls below the Photopicthreshold into Mesopic and Scotopic conditions. Thus, for good colorrendering and contrast, a white color should be projected throughout thePhotopic illuminance zone and may also be for the surrounding Mesopicillumination zone. However, in order to economize (for instance in orderto reduce the overall amount of LED light of a given hue which must beproduced and projected in peripheral areas of a target) the illuminatormay be allowed to project slightly non-white colors into the surroundingMesopic illuminance zone.

Although this Photopic threshold is commonly associated with a surfaceluminance of approximately 5 nit or greater, this can be translated to acorresponding "Photopic illuminance threshold" of approximately 30 luxfor Lambertian surfaces with 50% neutral gray reflectance factor. A 50%neutral gray Lambertian reflector is a suitable reference surface whichrepresents, in a statistical sense, a high percentile of actual objectsand surface to be illuminated.

Electronic control 122 energizes, protects, controls and manages theillumination of the plurality of LEDs 14, 16, and 114 through circuitry.Although those skilled in the art will understand that there are aplethora of electronic circuits which can perform substantially the samefunction, FIG. 21 illustrates the presently preferred circuit design foran automotive maplight.

Q1 and Q2 form a constant-current source. Q1's base current is suppliedby the microprocessor Port 0 through current limiting resistor R2. Q2regulates Q1's base current to maintain a substantially constant currentthrough R1 and hence the amber LEDs D1-D3. The regulation point is setby the cutin voltage of Q2's base-emitter junction. A detailedexplanation follows.

To energize LEDs D1 through D3, the voltage on Port 0 of themicroprocessor U1 must be raised. This causes a current to flow intotransistor Q1's base, I_(b)(Q1), to increase. This will cause thecollector current of Q1 to increase. Q1's collector current I_(c)(Q1)and the current through D1-D3 from the power supply VI are substantiallythe same as the current through R1. This is because Q1's emitter currentI_(E)(Q1) is equal to the sum of its collector I_(C)(Q1) and baseI_(B)(Q1) currents, and the base current is substantially smaller thanthe collector current (typically by a factor of 100). This can also bestated in equation form as follows (Equations 9-11).

    I.sub.E(Q1) =I.sub.C(Q1) +I.sub.D(Q1)                        9!

    I.sub.B(Q1) <<I.sub.C(Q1)                                    10!

    I.sub.E(Q1) ≈I.sub.C(Q1)                             11!

As the current through R1 increases, the voltage on Q2's base willincrease. Once Q2's base-emitter cutin voltage V_(BE)(Q2) is reached,Q2's base current I_(B)(Q2) will start to increase exponentially whichwill in turn cause an increase of in Q2's collector current I_(C)(Q2).Q2 will shunt current away from Q1 's base, preventing further increasesin Q1's collector current. The LED current is set at approximatelyV_(BE)(Q2) /R1 Ampere according to Equation 12.

    I.sub.C(Q1) ≈V.sub.BE(Q2) /R1                       12!

(approximately 36 mA at 25 degrees Celsius, with V_(BE) =0.68V and R=19Ohm). If the current through R1 should decrease for any reason, thevoltage across R1 will decrease, reducing Q2's base current and in turnits collector current. This will allow more of the current supplied bythe microprocessor U1 through R2 to flow into Q1's base which willincrease its collector and emitter currents. This will tend to returnthe R1 current and hence the current through D1-D3 to its originalvalue.

Because the emitter-base cutin voltage of a silicon transistor such asQ2 decreases at a rate of approximately 2.5 mV per degree Kelvin(ΔV_(BE)(Q2)), the current through Q1's emitter, collector and D1-D3will decrease at a rate of approximately (ΔV_(BE)(Q2) /R1) Ampere perdegree Kelvin (approximately 132 μA per degree Kelvin in this case).

Q3 and Q4 form another constant-current source. Q3's base current issupplied by the microprocessor Port 1 through current limiting resistorR4. Q4 regulates Q3's base current to maintain a substantially constantcurrent through R3 and hence the blue-green LEDs D4-D5. Operation ofthis current source is substantially the same as the current sourcewhich drives the amber LEDs (D1-D3). In the present design two currentsources are used to accommodate the different maximum current ratings ofthe blue-green and amber LEDs as well as to allow independent duty cyclecontrol and hence illumination intensity of the two colors. Someapplications may allow the use of a single current source or a simplecurrent limiting resistor and/or the series connection of the blue-greenand amber LEDs. Multiple current sources may also be required if theforward drop of a series string of the required number of LEDsapproaches the supply voltage too closely.

This temperature-dependent current drive allows the LEDs to be driven ator very near their maximum forward current during normal (cool)conditions and as the temperature rises there is no risk of overloadingthe LEDs. FIG. 22 shows the specified maximum forward current versustemperature for the preferred amber LEDs as well as the experimentallydetermined forward current versus temperature plot for the LEDs of thepresent invention in the above described circuit. As can be clearlyseen, the LEDs can be operated at approximately 42 mA at -40° C., atapproximately 36 mA at 25° C. and at 31 mA at 85° C. Thus the LEDs areoperated very near their maximum forward current at low temperatures,and the above described circuitry will automatically adjust the forwardcurrent of LEDs D1-D5 to stay within the specification as thetemperature rises up to 85° C. Thus, by utilizing the circuitry shown inFIG. 21, the output from the LEDs is maximized during periods of minimumambient light when the illuminator will predominantly be used (it istypically coolest at nighttime) and decreases during maximum ambientlight, i.e., daylight when the illuminator would not generally be used(it is typically hottest during the day). In order to maximize thebenefit of the LED illuminator and minimize the cost and complexityrequired to achieve that benefit, it is very important to operate themat or very near their maximum allowed current rating for the prevailingtemperature conditions.

One prior method for avoiding thermal overload of these LEDs whenoperated at high operating temperatures results was to permanentlyde-rate the LED to run at a non-varying current set at a lower levelwithin that specified for the LED at the maximum specified operatingtemperature, e.g., 25 mA at 80° C. However, this significantly reducedthe luminous output at low temperatures when the LED could be drivenharder (and have more output) without damaging the LED, if a morecapable circuit was employed.

Another prior method utilized a thermistor in the circuit which measuredthe ambient temperature and automatically de-rated the LEDs at highertemperatures; however, this complicated the circuit design and, moreimportantly, substantially increases the cost of the circuit.

The circuit of the present invention is a novel and inexpensive (andtherefore commercially viable) method of ensuring that the maximumallowed LED output is achieved at all operating temperatures andachieving nearly a 70% increase in luminous output at typical night-timetemperatures as compared to less sophisticated circuits.

LEDs have an operating current of approximately 30-70 mA which is muchlower than the typical incandescent lamp operating current ofapproximately 0.35 amps up to many amps. This lower operating currentallows the use of inexpensive bipolar transistors Q1-Q4, such as, forexample, MPSA06, for the LED drivers which are much cheaper than theMetal Oxide Semiconductor Field Effect Transistors (MOSFET) required inan electronic control circuit for an incandescent lamp. In addition todecreasing production costs, the bipolar transistors of the presentinvention automatically de-rate the LEDs as the ambient temperatureincreases.

As those skilled in the art will realize, the microprocessor U1 canmanage and manipulate the output from LEDs D1-D5. For example, byremoving any voltage from port 0 the base voltage to Q1 will be zero andno light will be emitted from D1-D3 and only light emitted from D4-D5will illuminate the interior of the vehicle. Similarly, the voltage fromport 1 can be removed and only amber light will illuminate the interiorof the vehicle. Of more practical importance is that the emission ofeither LEDs D1-D3 or D4-D5 can be modulated by the microprocessor U1 bymodulating the base currents to Q1 and Q3, respectively. Furthermore,the "amount" of amber light generated from D1-D3 relative to the amountof blue-green light (or other combinations if different LEDs are chosenin accordance with the above metameric teachings) can be varied simplyby controlling the modulation of voltage out of ports 0 and 1.Controlling this proportion is especially important in maplights becausethe properties of the vehicle interior, e.g., colors, dimensions, maywarrant a slightly different "white" color emission from the illuminatorto maintain maximum readability. By allowing pulse-width modulation, thepresent circuit design allows for modulation data to be stored innon-volatile memory U2 and easily changed depending on which automobilethe mirror assembly will be installed. In addition, time sequentialmultiplexing allows D1-D3 and D4-D5 to be turned on and off rapidly oneafter another, such that they are never actually on at the same time.The illumination produced in this fashion is still achromatic andeffective because the time constants for human visual response are soslow that the human eye cannot discern the rapidly changing color of theillumination projected by the color-complementary LEDs in theilluminator energized in rapid sequence. In the case of the present LEDilluminator, the on/off times can be very fast and the sequencingfrequency very high, because LEDs don't suffer from bloom timelimitations. Additive mixing occurs and therefore the light from theilluminator the vehicle looks white, even with a minor time delaybetween the presence of illumination from the two additive constituentsof the mixture.

In addition to manipulating color, the microprocessor U1 can pulse-widthmodulate the LED currents for purposes of thermal de-rating. Amicroprocessor U1 with internal or external temperature measurementmeans can modulate the LED currents to very precisely follow themanufacturers' specified current ratings at each temperature asillustrated by the curve in FIG. 22 labeled "Design Current forSoftware-Controlled Temperature Compensation". In the case ofmicroprocessor controlled thermal de-rating, the current limiting meansmust provide a current greater than or equal to the maximum of thedesign current for software control. For the amber LEDs D1-D3 in theexample in FIG. 22 the current limiting means must provide at least 48mA. This would require changing the value of R1 in FIG. 21 to 14 Ohm. At70° Celsius the microprocessor U1 would begin pulse-width-modulating thecurrent through D1-D3 in FIG. 21 to reduce the average current to safelevels using a lookup table, calculation or other means to determine thecorrect duty cycle. Alternatively R1 in FIG. 21 could be set to 10 Ohmfor a 68 mA drive current and the duty cycle set at 70% to maintain anaverage current less than the manufacturers limit. Obviously, there arean infinite number of current and duty cycle combinations that can beused to maintain the required average current as long as the peak drivecurrent does not exceed the LED manufacturer's peak current ratings.

The invention has been described in detail for a rearview mirrorincorporating an illuminator. However, those skilled in the art willrealize that the illuminator of the present invention may be used inother vehicular applications such as dome lights, vanity mirror lights,headlamps as well as engine and trunk compartment lights. A dome lightassembly or a vanity mirror assembly incorporating an illuminatoraccording to the present invention will have a housing, one or morelenses, and an electronic control in accordance with the aboveteachings. Any slight modifications to the housing, lenses andelectronic control will be clear to those skilled in the art. Inaddition to vehicular embodiments, the present invention may be used innon-vehicular embodiments requiring high efficiency, high reliability,long life, low-voltage compact, effective white light illumination aswell without diverting from the present teachings. Such applicationsinclude hand-held portable flashlights, head or helmet mounted lamps formining or mountaineering, task lighting for volatile environments whereexplosion is a hazard, illuminators mounted in difficult to maintainareas such as atop buildings, automatically activated emergency lightingor back-up lighting in commercial buildings, and microscope stageilluminators to name a few. Again, the minor modifications to thehousing, lenses and electronic control will be clear to those skilledand, therefore, it should be understood that these vehicular andnon-vehicular illuminator uses fall within the scope of the presentinvention.

While the invention has been described in detail herein in accordancewith certain preferred embodiments thereof, many modifications andchanges therein may be effected by those skilled in the art withoutmaterially departing from the novel teachings and advantages of thisinvention. Accordingly, all such modifications are intended to beincluded within the scope of this invention as defined in the followingclaims and, therefore, it is our intent to be limited only by the scopeof the appending claims and not by way of the details andinstrumentalities describing the embodiments shown herein. In theclaims, means-plus-function clauses are intended to cover the structuresdescribed herein as performing equivalent structures. Thus although anail and a screw may not be structural equivalents in that a nailemploys a cylindrical surface to secure wooden parts together, whereas ascrew employs a helical surface, in the environment of fastening woodenparts, a nail and a screw may be equivalent structures.

What is claimed is:
 1. An illuminator assembly, comprising a plurality of LEDs disposed on a support member such that, when at least two of said LEDs are energized, illumination exhibiting a first hue having and wavelength below 505 nm and projected from at least one of said plurality of LEDs overlaps and mixes with illumination exhibiting a second hue having a do said first hue and projected from at least one of the remaining LEDs of said plurality, where said overlapped and mixed illumination forms effective metameric white illumination.
 2. The illuminator assembly according to claim 1 where said first hue has a dominant wavelength ranging from about 476 nm to about 492 nm and said second hue has a dominant wavelength ranging from about 572 nm to about 605 nm.
 3. The illuminator assembly according to claim 1 or 2 where said metameric white light is defined by boundaries for white color translated from the revised Kelly chart and the CIE 1931 x, y, chromaticity diagram.
 4. The illuminator assembly according to claim 1 or 2 where said metameric white light is defined by the translated SAE J578 boundaries for white light.
 5. The illuminator assembly according to claim 2 where said first and said second hues have a locus of binary additive mixtures that are substantially coaxial with the Planckian Locus.
 6. The illuminator assembly according to claim 2 where said overlapped and mixed effective illumination has an illuminance at some predetermined distance from said illuminator greater than about 15 lux.
 7. The illuminator assembly according to claim 6 where said predetermined distance is greater than about 10 times the smallest dimensional distance between any two color complementary pairs of said plurality of LEDs.
 8. The illuminator assembly according to claim 2 further comprising an electronic circuit electrically connected to and operable to energize said plurality of LEDs.
 9. The illuminator assembly according to claim 8 where said circuit is temperature-sensitive, such that the current decreases as the temperature increases.
 10. The illuminator assembly according to claim 1 where said first hue has a dominant wavelength ranging from about 530 nm to about 572 nm and said second hue has a dominant wavelength ranging from about 420 nm to about 476 nm.
 11. A vehicle illuminator assembly, comprising a plurality of LEDs disposed on a vehicular support member where, when said plurality of said LEDs are energized, illumination exhibiting a first hue have a dominant wavelength below 505 nm and projected from at least one of said energized plurality of LEDs overlaps and mixes with illumination exhibiting a second hue adistinct from said fit hue and projected from at least one of the remaining energized LEDs of said plurality, where said overlapped and mixed illumination joins effective metameric white illumination.
 12. The illuminator assembly according to claim 11 where said first hue has a dominant wavelength ranging from about 476 nm to about 492 nm and said second hue has a dominant wavelength ranging from about 572 nm to about 605 nm.
 13. The illuminator assembly in claim 12, further comprising a electronic circuit electrically connected to a power supply and said plurality of LEDs and operable to energize and control the illumination of the plurality of LEDs.
 14. The illuminator assembly in claim 13, where said electronic circuit includes two independent circuits to control said at least one of plurality of LEDs exhibiting a first hue and said at least one remaining LEDs exhibiting a second hue.
 15. The illuminator assembly according to claim 12 where said metameric white light is defined by boundaries for white color translated from the revised Kelly chart and the CIE 1931 x, y, chromaticity diagram.
 16. The illuminator assembly according to claim 12 where said metameric white light is defined by the translated SAE J578 boundaries for white light.
 17. The illuminator assembly according to claim 11 where said first hue has a dominant wavelength ranging from about 530 nm to about 572 nm and said second hue has a dominant wavelength ranging from about 420 nm to about 476 nm.
 18. The illuminator assembly according to claim 11 where said first and said second hues have a locus of binary additive mixtures that are substantially coaxial with the Planckian Locus.
 19. The illuminator assembly according to claim 11 where said overlapped and mixed effective illumination has an illuminance at some predetermined distance from said illuminator greater than about 15 lux.
 20. The illuminator assembly according to claim 19 where said predetermined distance is greater than about 10 times the smallest dimensional distance between any two color complementary pairs of said plurality of LEDs.
 21. The illuminator assembly according to any one of claims 11, 15, 16, and 19, wherein said illuminator illuminates a portion of the interior of a vehicle.
 22. A vehicle maplight comprising:(a) housing; (b) a mirror element; and (c) a plurality of LEDs disposed within said housing on a support member such that when energized by a power supply illumination exhibiting a first hue having a dominant wavelength below 505 nm and projected from at least one of said plurality of LEDs overlaps and mixes with illumination exhibiting a second hue having a dominant wavelength about 530 nm, said second hue being distinct from said first hue and projected from at least one of the remaining LEDs of said plurality, where said overlapped and mixed illumination forms effective metameric white illumination.
 23. The illuminator assembly according to claim 22 where said first hue has a dominant wavelength ranging from about 476 nm to about 492 nm and said second hue has a dominant wavelength ranging from about 572 nm to about 605 nm.
 24. The illuminator assembly according to claim 22 where said first hue has a dominant wavelength ranging from about 530 nm to about 572 nm and said second hue has a dominant wavelength ranging from about 420 nm to about 476 nm.
 25. The vehicle maplight of claim 23 where said mirror element is prismatic.
 26. The vehicle maplight of claim 23 where said mirror element comprises a self-erasing electrochromic device.
 27. The vehicle maplight of claim 23 where said mirror element comprises a solution-phase electrochromic device.
 28. The illuminator assembly according to claim 23 where said first and said second hues have a locus of binary additive mixtures that are substantially coaxial with the Planckian Locus.
 29. The illuminator assembly according to claim 23 where said overlapped and mixed effective illumination has an illuminance at some predetermined distance from said illuminator greater than about 15 lux.
 30. The illuminator assembly according to claim 29 where said predetermined distance is 10 times the length of the largest dimensional distance between any two color complementary pairs of said plurality of LEDs.
 31. The illuminator assembly in claim 22, further comprising a electronic circuit electrically connected to a power supply and said plurality of LEDs and operable to energize and control the illumination of the plurality of LEDs.
 32. The illuminator assembly in claim 22, where said electronic circuit includes two independent circuits to control said at least one of plurality of LEDs exhibiting a first hue and said at least one remaining LEDs exhibiting a second hue.
 33. The illuminator assembly in claim 22 where each of said circuit decreases the current through said plurality of LEDs at a rate of approximately 139 μA per degree Celsius.
 34. An illuminator assembly comprising a plurality of LEDs disposed on a support member such that, when at least two of said LEDs are energized, illumination exhibiting a first hue having a peak wavelength below 505 nm and projected from at least one of said plurality of LEDs overlaps and mixes with illumination exhibiting a second hue having a peak wavelength above 530 nm, said second hue being complimentary to said first hue and projected from at least one of the remaining LEDs of said plurality, where said overlapped and mixed illumination forms effective metameric white illumination.
 35. The illuminator assembly, according to claim 34, where said first hue has a peak wavelength ranging from about 476 nm to about 492 nm, and said second hue has a peak wavelength ranging from about 572 nm to about 605 nm.
 36. The illuminator assembly according to claim 34 where said metameric white light is defined by boundaries for white color translated from the revised Kelly chart and the CIE 1931 x, y, chromaticity diagram.
 37. The illuminator assembly according to claim 34, where said first hue has a peak wavelength ranging from about 530 nm to about 572 nm and said second hue has a peak wavelength ranging from about 420 nm to about 476 nm.
 38. A vehicle illuminator, comprising a housing attached to the exterior of a vehicle, said housing having an opening adapted to receive a mirror element, said illuminator further comprising a plurality of LEDs disposed within said housing on a support member, such that when energized by a power supply, illumination exhibiting a first hue a peak wavelength below 505 nm and projected from at least one of said plurality of LEDs overlaps and mixes with illumination exhibiting a second hue having a peak wavelength above 530 nm, said second hue being distinct from said first hue and projected from at least one of the remaining LEDs of said plurality, where said overlapped and mixed illumination forms effective metameric white illumination.
 39. The illuminator assembly according to claim 34, where said first hue has a peak wavelength ranging from about 530 nm to about 572 nm and said second hue has a peak wavelength ranging from about 420 nm to about 476 nm. 